To validate the previously described models, measurements were conducted with Cam 1 and Cam 2 using identical SPDs. Therefore, luminance distribution measurements were conducted with Cam 1 and Cam 2, while simultaneously point luminance mea-surements were conducted as a reference. The luminance, using the cameras, was calculated according to the conventional method, theoretical optimization of crite-rion 1, and theoretical optimization of critecrite-rion 2. Eventually, the performance of the methodologies was compared with the point luminance measurements according to Equation 3.10.
3.3.1 Measurement setup
The measurements were conducted in a dark windowless room (4.4 m x 3.6 m x 2.7 m) containing a lightbox. The lightbox (Figure 3.2), measuring 1.2 m wide, 0.8 m deep and 0.8 m high, with a diffusely reflecting white painted interior (ρi= 0.85) was placed on a table in the middle of the room. The lightbox was consecutively fitted with three illuminants, an LED (Philips Smartbalance tunable white RC484B) at 4400K, a halogen (Philips IR 250CH 250W), and a fluorescent illuminant (Freshlight Pure light 32W), respectively, with properties according to Figure 3.1 and with lumi-nance ratios between the minimum and maximum lumilumi-nance within the entire scene of approximately 1:2250, 1:1400 and 1:1000, respectively. Baffles were applied for the halogen and fluorescent illuminants to prevent direct light on the camera sensor that might cause overflow.
400 450 500 550 600 650 700 Wavelength in nm
0 0.5 1
Relative radiant power [-]
Cam 1
400 450 500 550 600 650 700
Wavelength in nm 0
0.5 1
Relative radiant power [-]
Cam 2
400 450 500 550 600 650 700
Wavelength in nm 0
0.5 1
Relative radiant power [-]
SPDs
Figure 3.1: Spectral responsivities of the Red, Green and Blue channels of Cam 1 and Cam 2. Moreover, the SPDs of the LED (black), halogen (brown) and fluorescent illuminant (orange) are illustrated.
To indicate the effect of the illuminant’s SPD, the luminance should be measured with a minimum of spectral disruptions to preserve the original SPD. Therefore, 10 grey samples were applied because it is impossible to measure the luminance of the illuminant directly. The targets were successively placed at the back wall in the middle of the lightbox. Grey targets have a relatively uniform spectral reflectance, by applying 10 different samples we intended to limit the effect of the imperfect uniform reflectances. The reflectances of these samples were 0.12, 0.18, 0.26, 0.28, 0.38, 0.41, 0.43, 0.74, 0.90, and 0.93, respectively.
Point luminance measurements were conducted with a Konica Minolta LS-100 luminance meter with an accuracy of ±2% and a general V (λ) mismatch of 8%.
Moreover, luminance distribution measurements were performed with two camera sensors, Sony IMX219 (Cam 1) and Omnivison OV5647 (Cam 2), respectively, com-bined with a fisheye lens (FOV 187°) and single-board computer controlled over SSH
with an average accuracy ranging from ±5% to ±20% as indicated in Chapter 2.
Both devices were calibrated in advance. The camera settings were fixed and identi-cal for both cameras as far as possible (Table 3.1). HDR images were captured using the first measurement track of the Bee-Eye as illustrated in Figure 2.9. Overflow was automatically detected, although it did not occur due to the baffles. Post-processing was done using MATLAB r2017a and consisted of luminance calculations and ad-ditional calibrations. The LS-100 (±2%) and the luminance cameras were placed, side by side, at 1.5 m from the respective grey sample while being focused at the centre of the grey sample. The measurement setup is further elaborated in Figure 3.2 and Figure 3.3. The SPDs of the illuminants, shown in Figure 3.1 and Figure 3.4, were measured in advance of each individual measurement in the middle of the lightbox using a Konica Minolta CL500A illuminance spectrophotometer with an ac-curacy of ±2% and a general V (λ) mismatch of 1.5%. These SPD measurements also functioned as input for the theoretical model.
800 1500
1200800
Top View
Side View
Illuminant
Baffles
P3 P2 P1
Figure 3.2: Measurement setup with the sample indicated in Orange, P1-P3 represent the measurement positions of the three devices.
Table 3.1: Camera settings, including minimum and maximum exposure value (EV ).
Cam 1 Cam 2
Model Sony IMX219 Omnivision OV5647
Aperture (N ) f/2 f/2.9
Resolution 901x676 901x676
ISO 100 100
White Balance 1.3,1,1.3 1.3,1,1.3
EVmin 4.3 5.1
EVmax 18.8 19.4
Figure 3.3: Measurement setup with the three measurement positions aligned. The bottom images represent the luminance masks corresponding to the area measured by the LS-100 luminance meter.
3.3.2 Protocol
In total, 90 measurements were simultaneously conducted for all three devices. The measurements were conducted simultaneously because some minor variations were exhibited during the measurements with the illuminance spectrophotometer for the halogen illuminant mainly. Three positions (P1 – P3) were located at 1.5 m from the sample as shown in Figure 3.2 and Figure 3.3. To account for the potential differences between measurement positions the equipment was rotated such that each device has conducted a luminance measurement for each condition (10 samples and 3 illuminants) at all three positions. Hence, each condition was measured thrice, resulting in a total of 90 measurements.
3.3.3 Analysis methods
Each luminance distribution measurement provided one single HDR image to which a luminance mask identical to the opening angle of LS-100 luminance meter (Fig-ure 3.3) was applied. Based on the HDR image, the luminance was calculated for each illuminant, the conventional method and both optimizations, by applying the
respective r, g and, b weighting factors originating from the theoretical optimization found in Table 3.3. An individual photometric calibration (Equation 3.9, k) was ap-plied for each camera and method (conventional, criterion 1, and criterion 2). This calibration factor was developed such that the average measured results were equal to the average expected results for the conventional method, criterion 1 and, crite-rion 2, respectively. Subsequently, inferential statistical methods such as one-sided and two-sided unpaired student t-tests with a confidence interval of 95% were ap-plied. Moreover, Lin’s Concordance Correlation Coefficient (ρc) was used to indicate the accuracy and precision (variability) relative to the reference measurement (Sec-tion 8.2.6). In contrast to Pearson’s correla(Sec-tion coefficient, ρc is able to assess both precision (variability) and accuracy (bias) [176].