Helicopter NVG compatible cockpit illumination assessments

EIGHTEENTH EUROPEAN ROTORCRAFT FORUM

D - 09

Paper No. 83

HELICOPTER NVG COMPATIBLE COCKPIT ILLUMINATION ASSESSMENTS

Dr. H.-D.V. Bohm,

J.

Frank, S. Haisch

EUROCOPTER Deutschland GmbH

GERMANY

September 15 - 18, 1992

AVIGNON, FRANCE

1.8.92

EIGHTEENTH EUROPEAN ROTORCRAFT FORUM September 15-18,1992

Palais des Papes, Avignon, France

Helicopter NVG compatible cockpit illumination assessments by

Dr. H.-D.V. 86hm, J. Frank, S. Haisch

EUROCOPTER Deutschland GmbH, Munich, Germany

Abstract

During the last 12 years EUROCOPTER Deutschland GmbH (ECD) has worked on night vision aids including helicopter NVG (Night Vision Goggles) compatible illuminations. Different illu-mination have been studied and realized.

The requirements of MIL-L-85762A specifica-tion are very high and have to be checked with spectral photoradiometers. The level of NVIS ra-diance (NR < 1. 7x1 0-10 at 0.34cd/m2) has to be fulfilled for many instruments with NVIS (Night Vision Imaging System) "Green A" color in the CIE chromaticity diagram. Master caution and warnings have different chromaticity and radi-ance requirements. Military Helicopter (HC) mis-sions with dark ambient light levels (night level 5 according to FINABEL report with <0.7mLux) es-pecially need a NVIS-radiance performance value which avoids interference between the il-lumination and the response of Image Intensifier Tubes (liT). Sunlight readability of all instruments with good contrast at 100 000 Lux is additionally requ·lred.

ECD has a night VISion Laboratory (VISL) with different resolution test pattern and instruments like the Spectro-Photoradiometers PR 713/PC and PR 1980C/NVG and several high-sensijivity Photometers (Luxmeter). Additional test devices are a Philips NVG 3. gen. (Type I, Class AlB: 645 nm cut-on filter), an Integrated Helmet Systems (IHS, Type II), an ANVIS 3.gen. tubes (Type I, ClassA: 625 nm) and a Monoscope (2. Gen. tube and with Class A and Class B filters).

Experience was made with the PR 713 and PR 1980C on linear or logarithmic radiance scale re-garding different illumination technologies inte-grated into the HC cockpit:

o filtered pure green LED-Panels

o filtered incandescent light: e.g. green-filtered Bezels and postlights for 2" and 3" instru-ments,

o filtered LCD-MFDs, filtered CRT-MFDs and fil-tered EL-Displays,

o lighted switch or button technologies for sta-tus and warnings

o floodlight techniques with green-filtered goose-necks or UV -lamps with fluorescence from saturn yellow or blanc emeraude paint-in g.

The cockpit assessments on BK117/AVT and TIGER showed additional requirements for NV IS compatible illumination e.g. good legibility and readability, minimized masking effects (shadow-ing), high homogeneity of different lights, proper cockpit finish, reduced window reflections, instal-lation aspects of floodlights, minimized HC de-lectability from outside, no auto kinesis effects (hard contrast) etc. for a good NVG cockpit lay-out. These parameters were assessed by engi-neers and pilots with a questionnaire using a type of Cooper-Harper-Rating procedure.

The paper presents the problems, improvements after assessments on Primary Integration Rig (PIR) and results of measuring NVG radiance, luminance, chromaticity co-ordinates, colors and contrasts for different illumination techniques in HC cockpits.

1, Introduction

A pilot flying with a helmet-mounted NVG or Inte-grated Helmet System (IHS), compare chapter 2, must also be able to read the illuminated displays in the crewstations with his unaided, normal vi-sion (naked eye). Displays must be visible to the unaided eye while emitting very low levels of I R. If a display emits small amounts of radiant

en-ergy within the response range of the I

R-sensi-tive photo cathode, it may degrade the perform-ance of the NVG and therefore the visual acuity (compare chapter 4. Fig. 4-2). The flight safety of a HC should not be impaired. NVG compatible illuminated displays/instruments do not inter-fere with NVG sensitivity range and remain read-able by the naked eye, On the contrary during daytime with illumination level up to 1 00 000 Lux, a sunlight readability of all instruments is re· qui red.

The delectability of a HC cockpit illumination from outside view with IITs or the naked eyes should be minimized for an enemy.

There exist two solutions for making the HC cockpit lighting NVG compatible:

a) retrofit an existing cockpit or

b) new design of instrument illumination accord-ing to MIL-L-85762A.

At the beginning of the 1980"s a cost effective retrofit solution with floodlight electroluminescent (EL) and EL-Bezel technique (ref. 1) was selec-ted which had bad uniformity and which was not completely NVG compatible according to MIL. Remark: MIL-L-85762A was at

that

time

not

available.

The paper describes in chapter 2 the visual aids incl. night levels and the characteristics of MIL. Chapter 3 presents the VISionic Laboratory (VISL), chapter 4 measurements of NVIS resolu-tion and chapter 5 HC cockpit illuminaresolu-tion tech-nologies incl. measurements. Cockpit assess-ments on BK117/AVT and TIGER are mentioned in chapter 6.

Remark: HC exterior illuminations are not descri-bed in this paper.

2. Helicopter night flight requirements 2.1 Visual aids and night levels

In contrast to fixed wing aircraft, a HC flies lower. The so-called Nap of the Earth (NOE) flying is performed at heights between 3 and 50 feet above ground at a speed of approx. 0 to 40 knots at night and 0

to

70 knots in daytime. In the early days of aviation, information was restricted mainly to that directly coming to the natural sense organs like eye, ear and sense of balance. More and more artificial sensors are now being used, detecting signals of various wavelength which are translated into the area of natural per-ception. The pilot perceives most information with his two eyes.

Detailed descriptions of fundamental radiometric and photometric characteristics, NVGs, physiol-ogy, how the human eye works in photopic, me-sopic and scotopic vision can be found in stan-dard articles (e.g. ref.1,10,15,16) or books (ref.4). Visual aids, in the form of multispectral elec-tro-optic sensors for helicopter night flight ap-plication can be divided into three main catego-ries:

o !mag& Intensifier Tubes (liT, NVIS) near infrared (approx.0.6-0.93~m)

o Thermal Imager (TI) thermal infrared approx. 3-5 and B-13~m)

o Imaging Micro-Wave Radar's

micro-wave (approx. 35, 94 and 140GHz) (last sensor group will be installed in future HC's only)

The development of helmet-mounted NVG with 3rd generation liT for helicopter applications al-lows the pilot to look around the goggles into the

cockpit and onto the instrument panels in front of him with his "naked eye". He sees through the goggles into the outside world. With the 3rd gen. liT it is possible to better amplify ambient light from the night sky and foliage or a defoliated tree reflectivity (Fig. 2-2). c o 1E·02

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1E·05 { _ j _ L _ l 400 500 600 700 800 900 1000 wav11h•ngth lnml

Fig. 2-1: Spectral response of 3rd generation liT (unfiltered) and with Class A and Class B minus-blue filters in log.- lin. scale, compare chapt. 2.2.

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Fig. 2-2: Reflectivity for foliage or a defoliated tree and natural night-sky spectral irradiance in lin.-lin. scale (according to ref. 15)

The 3rd gen. liT uses an objective with an input window, a high sensitive GaAs!AIGaAs Photo cathode, a Micro Channel Plate (MCP), a P20 phosphor screen, an optical twister and an ocular with approx. 5cd/m2 _{output luminance, controlled} by an Automatic Gain Control (AGC) according to STANAG 4341. Observing the NVG screen, the observer receives a luminance level between photopic and mesopic vision, which means he detects the monochrome radiation with the cones (color) and rods (black and white) of his eyes.

Level of night % per year, illumination Level of %per year illumination obscurity conditions south France mLux ') obscurity northern mLux ')

Germany 1 very clear 14.0 40-200 1a 3 135-400 1b 10 40-135 2 clear 24.0 10-40 2 27 5-40 3 normal 7.0 2-10 3 9 2-5 4 dark 27.5 0.-2 4 19 1-2 5 very dark 27.5 0.1-0.7 Sa 22 0.5-1 5b 10 <0.5

Table 1: The table contains the distribution of the average ambient illumination and levels of obscurity (night) from a Finabel Study (1978) and measurements in northern Germany.

Remark: ') The night sky is observed by liT's and not by human eyes. The precise value should be measured in irradiance (W/m2

) instead of illumination (Lux) The values from table I were measured over a

period of one year in Southwest France and in north Germany respectively. The results indicate that for approx. 27.5% of the year very dark nights occur where illumination lies below 0. 7 mLux, so-called night level 5. 3rd gen. liT can amplify ambient light below 0.5 mLux.

Limits for image intensifier sensors in the Ger-man Army Aviation Branch are determined by o ambient light and

o transparency of the atmosphere.

Image intensifier vision of at least 1.5 km is cal-culated from those two parameters and the light value must not be less then 0.5 mlux.

On the contrary, a Tl sensor detects the emitted radiation of each body. The Tl introduces enhan· ced night vision capabilities for HC day and night flight together with new problems associated with the interpretation of visual information based on spectral and spatial characteristics differing from those provided by unaided vision. During a 24 hour mission the Tl sensor alone can have a great disadvantage. The absolute temperature characteristic or the emissivity of natural materi-als varies as a function of a 24 hour period. Thermal zero contrast (wash out effect) is ob· served especially during and immediately after rainfall, while the so-called cross-over effect is apparent at dawn and twilight. Then the fore-ground is not detectable against the backfore-ground. Therefore the combination of the two visual aids: liT and Tl, which are based on different physical principles, is better suited to fulfil the increased requirements of adverse weather conditions du· ring day and night time. An advanced Integrated Helmet System (IHS) with binocular vision incl. see-through capability (two CRTs, two liT's and two combiners with optical path on the helmet) can display head up images of the intensifier and thermal images superimposed with flight

symbol-ogy. The peripheral vision of an IHS is higher than in NVG, ref. 10 and 17.

Fig. 2-3: An IHS with see-through capability from GEC Avionics.

2.2 NVG compatibility requirements accord-ing to MIL-L-85762A

In order to fly night time missions with NVG or IHS, a helicopter cockpit has to fulfil some re· quirements concerning the cockpit lighting. These requirements and verification methods are speci-fied in the Military Specification MIL·L-85762A. The different possible techniques of cockpit lighting are freely selectable and are not de-scribed in MIL.

MIL-L-85762A classifies Gen 3 NVG compatible lighting according to Night Vision Imaging Sys-tem (NVIS) type and class:

Type I (NVG) Direct View Image:

The view through the goggle eyepiece includes only the NVG phosphor-screen image. The pilot must glance down to see his instruments.

Type II (IHS, Gat's Eyes) Projected Image: A combiner lens allows the pilot a see-through capability with a simultaneously projected NVG image

Class A: Addition of a 625 nm minus-blue cut-on filter to the objective lens (filter or dielectric coating) produces the response shown in Fig.

2-1. The one-percent (1 %) relative response is at 595nm, which allows cockpit illumination in blue, green (advisory) and yellow (caution, master caution, warning). To avoid interference from orange and red displays, MIL-L-85762A excludes them from Class A cockpits.

Class B: Addition of a 665 nm minus-blue cut-on filter to the objective lens produces the re-sponse shown in Fig.2-1. Here, the one-percent (1%) relative response is at 625 nm, which ex-cludes most of red from the liT response to allow cockpit illumination in blue, green (advisory), yellow (caution), and red (master caution, warning).

The German Army NVG (BM8043) uses a cut-on at 645 nm between Class A and Class B.

Chromaticity co-ordinates, total contrast and uniformity:

Sunlight consists of different wavelengths. A small part of these wavelengths is so-called vis-ible light detected by the three types of cones in the human eyes. White light is a mixture of the visible wavelengths (max.380·780nm). The dif· ferent colors neutralize each other. If a certain wavelength is filtered one obtains a dominant wavelength, the so-called color. The color pu-rity is determined by means of the Commission lnternationale de I'Eclairage (CIE, 1931) diagram with an improvement of the 1976 Uniform Color Scale (UCS, Fig.2·5). The purity is given in per· centage and is defined by the connecting line be· tween the white point and the periphery of the CIE diagram. The third dimension of color is added to the CIE chromaticity diagram by adding the luminance-factor axis (intensity, grey scale) which determines the color contrast. NVIS col-ors (Green A, Green B, Yellow and Red) are not standard aviation colors, but are particular to NVIS because of the limits on NVIS radiance (NR). For NVIS Class A goggles the red color is not allowed because red falls within the Class A NVIS response and would cause unacceptable NR. The measured UCS co-ordinates (u', v') are required to be within a color circle defined as (r)2 ;:>: (u'·ud2 +(v'-v1')2

The Index of Discrimination (ID) is calculated by means of the Pythagorean Theorem in a three dimensional space with the luminance contrast (IDL) and the color contrast (I DC).

ID = 'I/(IDL2+1DC2)

for sunlight and NVIS readability L1 logl2 IDL= _r;; IO\J\12 l1, l2 = luminance values of compared colors; L 1 > L2 IDC= D.u'2+.6.v'2 _0.027

t,u and t,v = 1960 CIE-UCS chromaticity co-ordi· nates of compared colors

Fig. 2·4: Total achieved contrast

NVIS CJU£H A I\IVII C«UN • HVII YUUtW

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Fig. 2-5: Chromaticity coordinates (u', v') UCS 1976 with MIL·L·85762A color levels

The uniformity of the HC cockpit markings shall be in a illuminance range of either 3:1 according to MIL-P-7788E or 2:1 according to MIL-L-85762A.

NV IS radiance (NR):

NR is a figure of merit to measure the degree of NVIS interference of illumination at a specified luminance. NR can be used to compare the rela-tive NV IS compatibility of illuminations. N R is calculated from the measured spectral ra-diance (W/sr m2 _{nm) of a light source.}

If the image of the cockpit lighting, when viewed through the NVG, is not brighter than the outside scene then that lighting could be considered as being compatible.

The most difficult terrain feature to see at night is a defoliated tree. The NVIS radiance of a defo-liated tree in starlight is derived by multiplying the spectral radiance of starlight by the spectral reflectivity of tree bark and using this curve for N(A.) in the equation, compare Figs. 2-1 and 2-2:

930

NRx

=

G(A.lmax fGA(),)•S•Nt(A.)•dA. 450

where

NRx = NVIS radiance threshold value of tree bark in starlight illumination (-); NRA for Class A, NRg for Class B response

G(A.lmax =standardization factor (1 mAIW), to obtain correct dimensions

GA(A) = relative spectral response for NVIS Class A (-)

S = scaling factor (-); (increase of device S/N ratio by taking higher luminance val-ues as 0.34 cd/m2_{; here 1)}

Nt(A.) = spectral radiance of tree bark in star-light (W/m2 _{sr nm)}

dA. =wavelength in 5nm increments

resulting in the radiance threshold value: NRt = 1.7 x 10·10 (-)

for NV IS with Class A cut-on filter.

Since the total amount of NVIS radiance emitted by a lighting component varies with the bright-ness setting, a luminance level had to be defined at which the NVIS radiance is to be measured. When using NVIS a pilot views his cockpit ligh-ting with his unaided eye. US Air Force tests showed that pilots using NVIS set their cockpit lighting to less than 0.1 foot-Lamberts (0.34cd/m2). The requirements for NRg only, in-cl. colors and luminances are shown in table 2. The equation for NRA and NRg in the MIL inclu-des a scaling factorS with S = Lr I Lm

where:

Lr = required luminance for NVIS radiance and Lm

=

luminance measured by the

spectre-radio-meter, to avoid S/N problems of the meas-uring device.

MIL-L-85762A describes further the following items: controls, secondary instrument lighting, compartment lighting, utility lighting, emergency exit lighting, caution and advisory signals, jump lights, work and inspection lights, CRTs, EL, LCD's etc. and also quality assurance provisions: first article and quality conformance inspections.

Color 1 976 UCS Chromaticity Coordinates NRg

u v radius

Green A 0.088 0.543 0.037 < 1.7 x 10-10 at 0.1fl

Green

B

0.131 0.623 0.057 < 1.7 x 10·10 at 0.1fl

Yellow 0.274 0.622 0.083 < 1.5x 1o-7 at 15fl

Red 0.450 0.550 0.060 < 1.4x 1o-7 at 15 fl

Table 2: Chromaticity Coordinates and NRg levels (extract from MIL-L-85762A, Tables VIII/IX)

3. ECD Night VISion Laboratory (VISL) In lhe ECO Night Vision Laboratory it is possible to conduct two kinds of measurements:

a) Resolution measurements of Image Intensi-fier Systems

b) NVG compatibility measurements according to MIL-L-85762A (radiance, luminance, chro-maticity coordinates, contrast etc.)

For both kinds of measurements a total darkroom is necessary. Total dark means not only a room without windows; the walls and the furniture are dim black, while the door is hidden behind a black curtain to keep out light coming in through the slits between door and floor. Every single light source e.g. light bulbs and LED's in the measuring-equipment or the computer is covered by black tape. To find all the disturbing light sources (or leaks) it is advisable to use a NVG.

Fig. 3-1a: The ECO Night Vision Laboratory (VISL)

For the Image Intensifier System measurements there is a defined and reproducible illumina-tionlirradiance source required.

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Fig. 3-1 b: Top view of the test set with two con-trollable integrated spheres in VISL to measure resolution and sensitivity of NVIS

Therefore two integrated spheres are in the labo-ratory installed which illuminate one end of the laboratory incl. target at an angle of 45° from the left and the right side (Fig. 3-1 b). It is therefore possible to achieve a very homogeneous illumi-nation of the end of the laboratory, where the vi-sion test boards e.g. the USAF 1951 high con-trast targets (1 00% or 70%) are located. The light sources in the integrated spheres are halogen spots (12 volts) operated in current-stabilization, to keep a constant color temperature.

For NVG and IHS tests the MIL-A-49425(CR) re-quires norm light A , which has a color tempera-ture of 2856 K (+/- 50K allowed). To simulate dif-ferent night vision levels without changing the color temperature, there is an adjustable Iris ap-erture in front of the halogen spot in the inte-grated sphere. Illumination levels between 0.01 mLux and 3.6 Lux can thus be reproduced. In the future starlight simulation experiments with a slide-projector are planned.

Fig. 3-2a: Integrated sphere

measured curve

69

49

Fig. 3-2b: Spectral radiance measurement of norm light A and theoretical Planck-curve for 2856K temperature

To measure sensitive illumination levels, the la-boratory is equipped with several high sensitivity photometers from LMT and EG&G. The LMT S 1000 has a measurint(l range from 600 klux down to 0.01 mlux (10- Lux).

Two Spectral Photoradiometers from Photo Re-search (PR713PC and PR1980C/NVG) and a Monoscope from Hoffman Engineering Company are used for the NVIS radiance and chromaticity measurements.

Figs. 3-3a: PR713PC fast spectro-photo radiometer mounted on a tripod

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Fig. 3-3b: Optical path from the Pritchard Aper-ture Mirror spectro-photoradiometer.

The PR713PC spectro-photoradiometer measur-ing-head functions by the Pritchard Aperture Mirror Principle:

The light passing through the objective lenses falls on a mirror, which is positioned at an angle of 45° to the line of sight. In the center of this Pritchard Aperture Mirror there is a small aper-ture. Only the light finding its way through this aperture falls on a diffraction grating. The spec-trally diffracted light is analysed by a 256-element photo detector-array. Light, which strikes the Pritchard Aperture Mirror, falls on a second mir-ror and then through the eyepiece. The image in the eyepiece shows the test object. The Pritchard Aperture is visible as a little black spot in the cen-ter of the image and characcen-terises the measuring area. A software programme run on a Personal Computer, calculates the sensor signals and shows the spectral scan, the CIE chromaticity coordinates and the luminance (approx. 1 - 23 000 cd/m2). See Figs. 3-3a and 3b).

The advantages of the PR713 are the mobility inside the laboratory or cockpit, the short measu-ring cycles of a few seconds to approx. 1 minute (maximum), but sources with low luminance le-vel(< 1cd/m2) cannot be measured. The spectral range is 390 nm to 1070 nm. The rapid decreas-ing sensitivity over 1000 nm of the sensor leads to noise, which is visible in the measuring plot. The PR1980C/NVG spectro-photoradiometer (Figs. 3-4a and 4b) consist of an optical head with a Pritchard Aperture Mirror Disc System for luminance measurements, a monochromator as-sembly for the spectral scans and a control and display unit. In contrast to the Pritchard Aperture Mirror System of the PR713PC the mirror is con-structed here as a revolving disc with several ap-ertures between 3° and 2'. The luminance meas-uring range from 0.01 e-5 to 1 0.00e7 cd/m2 _is much more sensitive than the range of the PR713PC. A typical ANVIS scan takes about one hour with 10 nm steps.

BEAM OlVERTINO Of>TICS

EHTRAHCE Sllf n10 ORO. SOOT OiBPEASION GRAllf .. Q MONOCitROMATOO ,t,$SEMBLY 8LICOH DETECTOR EIUl SLIT

Figs. 3-4a and b: The PR1980C/NVG spectro-photoradiometer and the optical path

For spectral scans the light leaves the optical head and, guided by two mirrors, enters the monochromator assembly, where it is dispersed by a double dispersion grating system. During a scan the gratings are turning simultaneously step by step. The steps are adjustable between 1 to 10 nm. One scanning cycle lasts about 1 hour, depending on the measured radiance levels of the source. The scan range lies between 380 and 930 nm for NVIS measurements. The software has additional features to the software of the PR713PC. For example, it is possible to choose a linear or a logarithmic scale on the abscissa, which is very important to show the near IR parts of a cockpit illumination. A further

remarkable feature is the so-called 'NVIS-Scan'. This is a spectral scan mode, in which the NV IS-radiance NRA or NRs according to MIL-L85762A is computed by the software. If NVIS-radiance and NVIS-color is in the allowed range, there ap-pears a 'pass' in the measuring-plot (see Fig. 5-1 c).

The monoscope (Figs 3-5a and 5b) is a special measuring equipment for the direct determination of the NV IS-radiance NRA and NRs.

Fig. 3-Sa: Monoscope NVG 103 from Hoffman Engineering Company

Essentially the monoscope consists of a second generation image intensifier tube with a phosphor screen. The gain is adjustable in four positions between 500 and 2000-times. Directly in front of the photo cathode there is a dimable LED. To measure the NVIS radiance of a cockpit light it is necessary to look through the monoscope. Then the LED, which is also visible on the phosphor screen, is dimmed to the same brightness as the cockpit light. A display shows the NVIS radiance, computed by the_ electronics according to the sensitivity of a 3. generation tube. The accuracy reached in tests is in +i-1 0% range, compared to the PR1980C. It is possible to mount a camera on the monoscope instead of an ocular lenses .

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Fig. 3-5b: Drawing and techn. data from monoscope

4. Resolution measurements of Image Inten-sifier Tubes (NVGs and IHS)

Three types of NVIS devices were tested. The Philips NVG (BM8043) is the benchmark for the NVIS: These goggles comprise two identical straight-through monoculars with fixed objective focus (approx. 10 m to infinity) and adjustable eyepiece focus. The objective is a 26 mm focal length, F-number 1.2 lens which gives a circular field of 42° and a magnification of 1 :1. The two monoculars are held together at the front on a tilting hinge for adjustment of Inter Pupillary Dis-tance (IPD) at the rear. Adjustment of IPD will vary the FOV overlap. The resolution measure-ment with several test persons (average values) is shown in Fig. 4-1. This NVG uses a cut-on til· ter at 645nm, that means between Class A and Class B.

The tested IHS from GEC Avionics (see Fig. 2-3) has a fixed objective and eyepiece focus. The field of view (FOV) of this prototype version is 35°. Both combiners are adjustable in three di-rections: I PO, distance to the eyes and in height. In contrast to the german NVG and the IHS the French NVG with 42° FOV from SOPELEM has adjustable objectives and oculars but no cut-on filters.

The resolution tests were made with a USAF 1951 bar pattern target. Several persons with a good visus (over 1 00%) had to determine at 9 il-lumination levels which bar group they can re-solve easily and which hardly. The mean of these both figures was averaged with values of the other persons. The cu!Ves in Fig. 4-1 are fitted to the average values of 6 persons.

0.01

_'"

il!umuuwon (mlux(

Fig. 4-1: Measurements of German NVG (42°, with cut-on filter at 645nm), French NVG (41 o, without cut-on filter) and IHS (35°, with cut-on filter) with four or six test persons

In order to demonstrate the degradation of spa-tial resolution of a NVG with non-compatible

0.01 0.1 10 100

target Illumination [mlux]

• 2.3 o 1.0 o 0.25 v 0.06 • 0.015 {W/sr•m ... 2] radiation levels of interference source

Fig. 4-2: Interference of a NVG with different levels of non-NVG compatible light. The ra-diation reached the NVG outside of the FOV and caused a remarkable degradation in the resolution

cockpit lighting, a resolution test was made in the presence of an interference light source (inte-grated sphere). Radiation from 600 to 950 nm was emitted and reached the NVG outside of the FOV. The results in Fig. 4-2 show clearly the impact of stray light as the source was placed outside of the FOV.

5. HC cockpit illumination technologies including measurements

5.1. NVG-compatibility measurements 5.1.1 Measuring problems and experiences

with a Spectro-Photoradiometer

Some problems may be encountered while mak-ing NV IS compatibility measurements:

o small areas (letters or pointers) cause a bad SIN ratio for the PR and subsequently iden-tify an NVIS compatible item as not compat-ible, because the noise floor is integrated and seen as NR radiance. As much light as pos-sible must be trapped (larger aperture or il-lumination level higher than the 0.1 fl, see ref.15, for more detailed discussion of PR problems)

o it should be measured that the spectral beha-viour of the component remains unchanged at higher levels

o light/radiation sources or stray light in the darkroom should be prevented

o the interpretation of a scan is difficult when plotted in log ordinates

o pass or fail statements should be carefully examined (calculation of NRA or NRs. NR limits, noise floor from the machine etc.) There are NVG on the market without a clear de-finition of their response. The NVIS class A or B rules only lead to the desired compatibility toge-ther with a properly filtered NV IS device.

5.1.2 Measurements on NVG compatible equipment

Some major components of a NVG compatible crew station are shown. The results from NVIS tests are accompanied by a short description of the item.

Internal illuminated panel (Class B compat-ible): Made from an acrylic plate and filtered pure green LED's, as shown for a real TIGER sample in Fig. 5-1 a. The dimming curve is com-parable to those of incandescent lamps. The supply voltage range is 14 to 28VDC and the

nominal illuminance is 1 fL (3.4 cd/m2

) at 28VDC. The NRs value is well under the limit, the color is at the border of the NV IS Green 8 area because of the color shift by the saturn yellow paint. This fluorescent paint allows a second mode by night: illumination with UV floodlight.

Cross sett10n A- B of NVIS·-<::ompaltble LED Panel (not to scale)

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Internal illuminated panel (primary HC illumination)

a) Cross section of filtered LED panel, comp. Fig.5-1b (according to ref.15) b) Sample of a panel with filtered LED (560nm,from COMTRONIC); UV-floodlight

fluorescence is possible on saturn yellow paint

c) Logarithmic spectral radiance scan of such a LED panel radiating through the saturn yellow paint

Internally illuminated aircraft clock (Class A compatible):

This is a conventional instrument with an internal filtered incandescent lighting and a wedged cover glass. NRA is quite low, the color is right in the center of NVIS Green A. This clock may be used in a class B cockpit also but with a slight color shift if desired. At rated voltage the lumi-nance is at the upper limit (1.5 fl).

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b) spectral radiance measurement of the light with a logarithmic NR presentation c) USC 1976 chromaticity diagram

Color MFD with LCD: Two color MFDs with LCD technology are installed in each TIGER cockpit. They have 512 by 512 quadruples with 2 green, 1 blue and 1 red pixels for color genera-tion. Fig. 5-3a shows the spectral radiance scan for white color without a special NVG filter. The amount of NR is unacceptable. By adding a Schott BG39 filter the NVIS compatibility is ob-tained according to MIL with NRs < 2.2 x1 o-9 for

white, compare Fig. 5-3b and NRs < 1.1 x 1 o-8 for red. The MFD has not yet been tested by the pilots during a night flight mission to confirm ac-ceptance compatibility. The chromaticity diagram, Fig. 5-3c, shows the white point (little cross) and the three pure colors red, green and blue which are extracted by three scans. The blue color should be enhanced.

Figs.: 5-3a, b, c:

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UV

illumination as secondary lighting with saturn yellow. The measurement set-up as well as the test results for this lighting method is shown in Fig. 5-4

In the TIGER there is a group of standby instru-ments in the pilot's cockpit which has an inde-pendent

UV

illumination. Their dials are painted

Fig. 5-4a: Fig. 5-4b: UV 11oodligl1t -~ UVIight _____ Light emitted by flourescence PHOTO RESEARCH PR 1980 Cll'NG

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Secondary instrument and backup floodlighting

The secondary instrument and display floodlighting system of the TIGER cockpit is made with pure green LED's and filter glass. The color is NVIS Green B, the NR has a pass for Class A and Class B. The measurements were made against a white standard.

Fig. 5-5a: Secondary instrument and backup floodlighting of TIGER cockpit with pure green LED and filter glass (from COMTRONIC)

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5.2 Open areas in NVG cockpit illumination o Red flags and red range markings on dials

must be painted with fluorescing red paint in order to be seen under NVIS compatible green light. Otherwise they are invisible. There is only a black space, see Fig. 5-7 a. Lit with light, e.g. ANVIS Green B, free of IR this generate an IR radiation seen by the NVG. The allowed amount of NR of this type is not covered by the above mentioned spec. o For a flag, which appears only for the

appro-priate event, we could assume an NR as de-fined for NVIS Red. According to FAA rules for the standby instruments ( e.g. torque, RPM, Temp.) however all range markings emtt NR.

There is a very simple and effective solution for this problem by changing from the color coded range markings to form coded range mark-ings. This problem is considered in STANAG 2324 but there is a long way to go in order to ob-tain FAA civil certification. For military aircraft there is more freedom and Fig. 5-7b shows our proposal for a form coding.

Fig. 5-7 a: Range markings with color coding: un-der green light there are no more clear cues vis-ible,

o Flashlights are also not covered by the above mentioned spec. It is desirable to have an NR limit for flashlights with an amount of white light which is used for map reading.

o There should be a recommendation for a ma-ximum of totally illuminated yellow and red sutiaces from indicators, push buttons, warn-ings etc. APACHE, an American supplier, proposes an area of about one square inch as

the upper limit. This is also valid for yellow, red and white symbols on MFD's - many sym-bols drawn with those colors will disturb the NVG.

Fig. 5-7b: Rang markings with form coding (instead of color coding)

6. Cockpit assessments on BK117/AVT and TIGER

ECD has been working on NVG compatible cock-pit illuminations since 1984. At that time precise requirements of luminance/radiance in the res-ponse range of the liT's were not established. A lot of work was done by retrofitting conventio-nal instruments in existing HC cockpits, e.g. 80105 and BK117. The 3. gen. NVG type incl. their exact filter characteristics are prerequisites for the right illumination techniques design.

6.1 BK117/AVT Cockpit lighting (retrofit) AVT {Avionik Versuchsausrustungs Trager) is a flying test bed helicopter, based on BK117, to ob-tain experience with some TIGER avionic sys-tems and missions. Trials are planned with NVG, IHS, FUR, DMG, CDU, MFD, Data transmission, GPS etc. MMI aspects will be studied addition-ally. NVG compatibility of the AVT cockpit is a prerequisite for trials with NVGs or IHS. The goal was to achieve, by retrofitting, a quality according to MIL-L-85762A, Class B.

The philosophy applied was:

a) to keep the standard internal white lighting system alive (for conventional civil certification and flight rules)

b) for a NVIS flight: switch off the white lighting and switch on green floodlights, put filter glas-ses over the annunciator panel and the 2 ma-ster caution warnings

c) to change the internal illuminated panels into pure green LED panels

d) to filter the colored beam index CRT-MFDs 83- 16

The following components are integrated into AVT:

green filtered incandescent (GFI) floodlight, GFI goose necks, GFI post lights, GFI utility lamp, indicators from Staco, filters from Wamco and Schott, CRT-MFD, EL-Display and LED keys of CDU.

There are exceptions like the compass, some il-luminated push buttons etc. which have been replaced by new ones. They are now NVG com-patible even in a normal civil night flight.

Examinations of the single components accord-ing to MIL-spec. have shown good results in the VISL and the pilots gave a positive assessment. There are some effects like fluorescent paint on flags or range markings which may be seen with the NVG but there is no NR limit defined in the MIL-spec. (see also chapter 5.2).

The most difficult items to replace are small round indicators (around 1 Omm diam.) with yel-low or red caps. There is no supplier which gua-rantees on one hand sunlight readability and on the other hand NVG compatibility. Large size in-dicators from, for example Eaton or Kerry, should be installed, but a given !-panel cannot accom-modate such large items.

Figs. 6-1 a and 1 b: NVG compatible floodlight illu-mination on BK117/AVT

6.2 TIGER Cockpit illumination assess-ments; new design

The TIGER has a tandem cockpit for the pilot and the gunner/copilot with few conventional in-struments. New instruments are: Pilots and gun-ner visionic and armament panels, EL-CDU, LCO-MFD, RFI, etc. The primary illumination of the TIGER cockpit design took into account UV floodlighting technique with fluorescence from saturn yellow paint and integrated LED panel lights. Floodlight techniques, especially in a nar-row cockpit, lead to shadowing and installation problems. The amount of lights using each tech-nique and other lighting aspects have to be as-sessed.

Therefore in the last three years several TIGER Cockpit illumination assessments were made in the VISL, on the Class II Mock-up and on the Primary Integration Rig (PIR). The following pa-rameters were assessed by engineers and pilots using a questionnaire in a Cooper-Harper-Rating procedure for the pilot and the gunner cockptt: o NVIS compatibility of illumination,

o good legibility and readability with adapted illumination brightness (dimming range), o minimized masking effects (shadowing), o installation/mounting aspects of lamps on

glare shields and side consoles . o high uniformity/homogeneity (2:1) of dtfferent

illuminations,

o proper cockpit finish, o reduced window reflections,

o minimized HC-detectability from outside with naked eyes and with NVG's

o no auto kinesis effects (hard contrast) for a good NVG-cockpit layout, etc.

Following improvements from the previous vis-ual assessments have been taken into account for final solution:

o

Glare shields extended to prevent mirror ef-fects and reflections in the front, roof and side windows especially from illumination from the side consoles

o

LED panels in both cockpits upgraded for ho-mogeneity and NVG-compatibility (MIL-L-85762A)

o

Location of UV lamps and upgrading for uni-formity/homogeneity and NVG-compatibility o Position areas of green lights and high

inten-sity to allow back-up function on legend

legibi-lity .

o Handlight available with Green and UV ltght o Optimization of brightness (dimming curves)

setting range for the different illuminations o Brightness and color optimization on status

and warning lights

o NVG compatibility of EL- and filtered LCD-Displays

Phase lntegr.

uv

Green Approx. light

LED flood- flood levels under

liqht liqht liqht tent (mLux)

0

Familiarization with the equipment X X X 100

Examination of discrete X 100

1 components X

"one after the other" X

2

Operational modes X X 50- 1000

without NVG X X

Operational modes with German NVG incl. NVG X X 0.5 -50

compatibility 1) (discrete and mixture with green

3

X X

liqht)

X X 0.5- 50

Operational modes with French NVG incl.

4

NVG compatibility 1) (discrete and mixture with X X

I

green liqht)

5

Delectability from outside lntegr.+ Green X X 0.5- 50

UV +Green X X

German NVG: Phili s BM8043 French NVG: So elem OB56A p p 1 J sub·ectlve and quahtat1ve tests

Table 3: Procedure of (visual) cockpit lighting assessment; there are 2 assessments: one for the pilots and one for the gunners cockpit

The result of the TIGER illumination assess-ments is the a mixed solution:

o The main primary illumination on all side consoles, inter pedestal consoles and the in-strument panel will be with integrated LED panels.

o Excepted areas will be illuminated via indirect UV lighting under glare shields with fluores-cence effect coming from saturn-yellow (or blanc emeraude) painting (Fig. 5-1a). These areas are, for example the back-up-instru-ments, the clock, etc. on the front instrument panels, and for the government furnished equipment (e.g. IFF) on the gunner right hand console, compare Figs.6-2a and 2b.

'"

0

PJ t Research CALCULATED DATA

PHO. REFLECTANCE = 8. 596e+tUU. X RAD.· REFLECTANCE = s.545e+99.1 X

SAMPLE FILE / SOURCE FILE

•

26

•

6

o A secondary green floodlight will be instal-led additionally under glare shields and side consoles to reduce hard contrast and for back-up purpose.

o A moveable hand lamp with green and UV light and with a rechargeable battery is sto-wed in a box (for each cockpit).

The helicopters front windows are in the optical path of the NVG. Therefore we have carefully looked at the transmittance of the material. Fig. 6-1 shows the basic transmittance measuring set-up and the results obtained.

c opyl"Ig I J t< c > 19BB .1991

-DATA TYPE Spe-etri\View Cie i976

REFLECTANCE Vex-sioo U2,12 ciE 1931

GRAPU MODE PR-713/PC ZooM

DUAL FILE S/H 2461 Plo."int

DtlTE 91 Jun 92 pLot

TIME 16:37:14 Bin store

SPECTRAL REFLECTANCE PEAK

..

399

""

HEA)) PAAAHETERS 199 AP: n.a. BW: 19 hM ---.~~ lt:

•••

Msec

..

. 69 c-!:_

'

--

-~

..

_~-'jitP"- . -·-

_'

_'

--- C\": 1 ~

..

"'rrn'- _{USED ACCESSORYrs} z + 0

•

Standar,l ObJe ~

..

.

..

~

..

!;!

..

...

~

49-"

;::

•

'

- -

LASTH~S:

•

224 ~ _{SPS p p}

••

~

..

20

..

..

o; ...

...

SAVEH~S:

•

1

•

etns nM 759

Fig. 6-1a: The transmittance of the TIGER front windows is better than 80% in the visible and liTre-sponse spectrum

Fig. 6-1b:

l1crn undo Uiffcrcnl <~ngks·

/ 90.45 rkgr. ~nd

l

rt'al TI<JEI~ po~otion~

''"'"

~"'u

2856

K~

I ///

0

~- ---~~/

----~

...-:· Photo/R.adnHHctcr 390- 1070 nm

/

"

TIGER front window (outside) (inside)

Principle of transmittance measurement of a TIGER front window

=<· '·~»·'"'''

- - - <>:v

~--Fig. 6-2a: TIGER Pilot-Cockpit, design drawing _{Fig. 6-2b: TIGER Gunner-Cockpit, design drwg.}

Fig. 6-3a: TIGER Pilot-Cockpit _{Fig. 6-3b: TIGER Gunner-Cockpit}

PIR (primary Integration Rig)

used as an aircrew station lighting mockup Board wi1h USAF 1951 pattern ---oott

IIIII

IIIII

IIIII

=

illumination level

variable illumination source 2856 K

Darkroom made by mobH tent Residual illumination < .01 mlux

from 0.5 to 200 mlux -rasp. irradiance

Fig. 6-4a: Dark tent over TIGER-PIR with target (bar chart) and integrated sphere for lighting assessment

Fig. 6-4b: Black tent over cockpit section (PIR)

7. Conclusion

The VISL has proven to be an excellent, well-equipped facility providing an ideally suited envi-ronment for the assessment of, in particular, HC cockpit lighting. It can equally be ~~ed

.tor

custo-mer service to make tests and venfication proce-dures according to MIL-L-85762A.

The primary cockpit lighting for a modern night mission HC with filtered pure green LED techno-logy is a very good and cost effective solution .. The results of various tests of NVG compatible components show the possibility of making. a really NVG compatible HC crewstation complying with MIL-L-85762A requirements.

8. References

1. H.-D.V. 86hm, "The Night Vision Goggle Compatible Helicopter Cockpit"; Tenth

European Rotorcrafl Forum, Paper No.40, The Hague, The Netherlands, Aug. 28-31, 1984

2. H.-D.V. 86hm: Review: "FUR, NVG and HMS/D Systems for Helicopter Operation", AGARD, Conference Proceedings No. 379. The Aerospace Medical Panel on Visual Pro-tection and Enhancement, Athens, Greece, April 22-24, 1985, Paper No.2

3. M.A. Veve, Ph. Stuckelberger, "Helicopter Cockpit Organization for Night Vision", Elev-enth European Rotorcraft Forum, Paper No. 16, London, England, Sept. 10-13, 1985 4. I.P. Csorba, "Image Tubes", Howard W. Sams

& Co., Inc., A Publishing Subsidiary of ITI,

4300 West 62nd Street, Indianapolis, Indiana 46268 U.S.A., 1985

5. G.F.H. Lloyd, "The UK NVG compatible Cockpit Lighting Standard", and "Cockpit Lighting Standards and Techniques for use with NVGs", 1986, RAE FS(F), Working Paper No.6 and "A brief guide to NVG compatible Cockpit Lighting", Royal

Aircraft Establishment report, Farnborough, Feb. 1987

6. M.S. Scholl, J.W. Scholl, "Cockpit Readiness for Night Vision Goggles", SPIE Vol. 778 Dis-play System Optics (1987). p. 54-60

7. F. Reetz Ill, "Rationale behind the require-ments contained in military specification MIL-L-85762 and MIL-MIL-L-85762A", Report No. 83-20

NADC-87060-20, Naval Air Development Center, Warminster, PA, 17. Sept. 1987 8. MIL-L-85762A, "Lighting, Aircraft, Interior,

Night Vision Imaging System (NVIS) Compat-ible", 26.8.1988 (includes STANAG 3800) 9. D. Evans, "LED Displays and Indicators and

Night Vision Imaging System Lightung", Hewlett-Packard booklet, Application Note 1030, 11/89

10.H.-D.V. B6hm, R.Schranner, "Requirements of an HMS/D for a Night-Flying Helicopter", SPIE Vol.1290 Helmet-Mounted Displays II (1990), p.93-1 07

11.C.E. Rash, R.W. Verona, J.S. Crowley, "Human Factors and Safety Considerations of Night Vision Systems Flight Using Thermal Imaging Systems", SPIE Vol. 1290 Helmet-Mounted Displays II (1990), p. 142·164 12.C.E.Rash, R.W.Verona, "Compatibility of

air-craft cockpit lighting and image intensification night imaging systems ", Optical Engineering, 29(8), August 1990, p.863-869

13.G.F.H. Lloyd, "Night vision using Image In-tensifiers", Shephard Conferences, Night Vi-sion 90 Conference Proceedings, No.2, pp.1-17,30.Aug. 1990

14.H. Hesse, "NVG training in the German Army", Shephard Conferences, Night Vision 90 Conference Proceedings, No.4, pp.1·22, 31. Aug. 1990

15.G.W. Godfrey, "Principles of Display Illumina-tion Techniques for Aerospace Vehicle Crew Stations" Aerospace Lighting Institute, Tampa, Florida, revised 1991, Third Editioo 16.L. Forsberg, K.-G. Fransson, "About NVG,

Light and Colour Measuring Methods Experi-ences", FMV, Sweden, Swedish Defence Ma-terial Admioistration, Electronics Laboratory, Unk6ping, Jao. 1991

17.H.-D. V. B6hm, H. Schreyer, "Integrated Hel-met System Testing for a Nightflying Helicop-ter", Seventeenth European Rotorcraft Forum, Berlin, FRG, Sept. 24 -26, 1991, Proceediogs of Deutsche Gesellschaft tor Luft- und Raum-fahrt e.V. (DGLR), Godesberger Allee 70, 5300 Bonn 2, Germany and SPIE, Vol.

1456,(1991 ), p.95-123

18.H. Hellmuth, H.-D.V. B6hm, "Equipment, more or less ready to be used in Helicopters", AGARD, 63rd Avionics Panel Meeting/Sym-posium on "Advaoced Aircraft Interfaces: The

Machine Side of the Man Machine Interface" held in Madrid, Spain, May 18-22, 1992 19.Korry booklet, "The leader in NVIS-compatible

lighted displays"

20.STANG 4341, High Performance liT with GaAs Photocathode for Aviation NVGs, 1988. 21.MIL-I-49428 (ER), Image Intensifier Assam·

bly, 18 mm Microchannel Wafer, MX-10160/AVS-6, 18.Jan.1985

22.P. Michlein, "Beleuchtuogstechniken im Hub-schraubercockpit und ihre Vertraglichkeit mit

Bildverstarkerbrillen der 3. Generation", Dip\omarbeit, ECD/Fachhochschule MCmchen, Okt. 1990

23.K. Freundorfer, "Nachtsichtkompatibler Testaufbau zur automatischen und homage· nen Dimmung von Hubschrauber-Cockpitin-strumentenbeleuchtungen", Diplomarbeit, ECD/Fachhochschule MOnchen, April 1991 24. S. Haisch, "Auflosungs- und Rauschgrenzen·

bestimmung von Bildverstarker-Brillen, Beur-teilung der BiV -Kompatibilitat verschiedener Cockpitbeleuchtungstechniken im Hubschrau-ber", Diplomarbeit, ECDIT echnische Univer-si\at MOnchen, Aug. 1992

25.Booklets or manuals: Apache, Claropan, Comtromic, Eaton, ECE, EEV, EG&G, ETS, Hoffman Engineering, GEC, HP, INSCO, ITT, Korry, Litton, LMT, Lucas, Lumitec, Oxley, Philips, Photo Research, Rohde & Schwarz, Schott, SELA, Sextant, Siemens, Sopelem, Staco, VDO, Wamco