Technical Report Spectral Sensitivity and Transmittance Measurements of a Sinar 86H CTM Dual-­‐RGB Digital Camera Roy S. Berns Yixuan Wang 30
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Executive Summary The spectral sensitivity and spectral transmittance factor of the sensor and filters of a Sinar CTM (color to match) digital camera system were measured using a calibrated monochromatic source and spectrophotometer, respectively. The measured spectral sensitivities agreed well with published data. However, the measured relative quantum efficiency of the blue channel was about 15% lower than the published data. The spectral transmittances of the two filters matched expected absorption values. However, the coatings on the blue-­‐green filter produced excessive spectral selectivity (“ripple”). Comparing the modeled and measured camera signals for an Xrite ColorChecker Classic validated accuracy. Introduction It was of interest to measure the spectral properties of the Studio’s high-­‐resolution Sinar CTM imaging system. These data can be used to build a camera model, prototype filter-­‐
based enhancements such an NIR imaging, and spectral estimation, among others. Experimental Apparatus An Oriel incandescent light source, Oriel single-­‐grating monochromator, Oriel filter wheel (for second-­‐order dispersion rejection), and 4” integrating sphere were arranged to produce monochromatic light with a 13nm triangular bandpass (Figure 1). The effective spectral range was 380nm – 750nm. Figure 1. Apparatus to produce monochromatic light.
2 The Sinar camera system consisted of a 86H camera back (Dalsa 48MP full frame sensor FTF6080C), rePro camera body, eShutter, HR 100mm lens, Dual-­‐RGB filter slider, and LC slider for focusing (figure 2). This particular camera does not have an IR blocking filter in front of the sensor; instead, it is clear glass. When measuring spectral sensitivity, the Dual-­‐RGB filter slider was removed. Figure 2. Sinar camera system.
A Minolta CS-­‐2000 spectroradiometer was used as a transfer standard. The monochromatic light was measured using the Minolta following image capture. Three successive measurements were averaged. The Minolta was used to calibrate the monochromator wavelength scale and record spectral radiance. The spectral radiance of each measured wavelength is plotted in Figure 3. 3 30
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Figure 3. Spectral radiance of monochromatic light from 380nm to 750nm in 10nm
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A Macbeth ColorEye 7000 was used to measure the spectral transmittance factor of the two filters comprising the Dual-­‐RGB filter slider. Camera Set Up and Imaging The quantum efficiency of the Dalsa sensor (shown in their technical literature) was used to identify the wavelength that would produce the largest signal: 600nm. The exposure time and ISO (amplifier gain) were adjusted to achieve a 14-­‐bit signal near 15,000 for the image of 600nm: 4s and ISO 100 for an f5.6 aperture. Measurements, at first, were taken every 20nm to insure the settings were reasonable, particularly at short wavelengths where both the sensor and light source have low output. There was also a comparison between including and omitting the order-­‐sorting filters. These preliminary data indicated that the order-­‐sorting filters had a negligible effect on the results and could be omitted, thereby increasing radiance at short wavelengths (as plotted in Figure 3). The experiment was repeated at 10nm intervals. A second experiment was performed between 380nm and 450nm at much greater exposure time and gain to validate these wavelengths. The data were very similar and the results from the main experiment were used in order to have a single set of camera settings. Thumbnail images are shown in Figure 4. The images were stored as linear 14-­‐bit DNG. The average signal of the center 412,827 pixels was used to define the camera signal for each wavelength, stored as floating-­‐point data (Figure 5). 4 Figure 4. Camera images of the monochromatic light.
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Figure 5. Average camera signals as a function of wavelength. 14 bit data were
converted to floating point data and scaled between 0 and 1.
Spectral Sensitivity Results and Discussion Spectral sensitivity is calculated from the camera signals (Figure 5) divided by the spectral radiance (Figure 4), wavelength by wavelength. The results are plotted in Figure 6. The published data by Teledyne for this Dalsa sensor is also plotted in Figure 6. Both data were normalized to have the identical peak sensitivity for the red channel at 600nm. The main 5 discrepancies were a lower blue sensitivity and the red and green channels showing a rise in sensitivity at shorter wavelengths. The decrease in blue sensitivity mainly affects noise. Typical of all Dalsa color sensors, the green channel peaks at 530nm. For highest color accuracy, the peak should be near 555nm, the peak of the human visual system’s luminance response. 1
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Figure 6. Spectral sensitivity of the measured Sinar camera (light lines) and published
data (dark lines) for a Dalsa FTF6080C (from datasheet 20120808),* each normalized to
the peak sensitivity of the red channel at 600nm.
The rise in sensitivity was a result of measurement uncertainty using the Minolta spectroradiometer. The radiances at these wavelengths were very low (Figure 3) and any imprecision had a dramatic effect because of the division. An assumption was made that the sensitivity of the sensor reduced to 0 at 380nm. The measured data were adjusted to trace the datasheet spectra and smoothly reduce to 0 at 380nm, the results plotted in Figure 7. The measured data are very similar to the published data. The final data for the Studio’s Sinar camera are tabulated in Table I at the end of this report. * Note that there is a more recent datasheet: 20130710. The sensitivities are much different than those published in the 20120808 datasheet. 6 1
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Figure 7. Spectral sensitivities of the Sinar CTM camera system (light lines) and
published data (dark lines).
Transmittance of Filters and Discussion The spectral transmittances of both filters are plotted in Figure 8 and tabulated in Table I. They are each an absorption filter with a hot-­‐mirror coating to achieve a visible bandpass filter and an anti-­‐reflection coating on the side facing the camera lens. Hot mirrors often produce spectral “ripple” and the transmittance of the yellow filter is typical. The blue-­‐
green filter’s excessive ripple was a surprise. 7 1
Transmittance factor
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Figure 8. Spectral transmittance factor of the two filters in the Sinar filter slider.
Testing Data Using Measured and Estimated Camera Signals An Xrite ColorChecker Classic was imaged using the Sinar system with Broncolor strobes aimed at 45° from the normal. The measured spectral reflectance factor data for the chart were used to calculate camera signals using CIE illuminant D55 as an approximation to the strobes (model data). Both the model and measured data were normalized to unity for the white patch of the ColorChecker. The results are plotted in Figure 9, verifying the accuracy of the measured spectral sensitivities and filter transmittance factors. 1 Measured data 0.8 0.6 y = 1.0057x -­‐ 0.0006 R² = 0.99862 0.4 0.2 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Model data Figure 9. Scatter plot of model and measured data for the Xrite ColorChecker Classic.
8 Table I. Spectral sensitivity and transmittance data.
Red Green Blue Wavelength channel channel channel Cyan Yellow 380 0.000 0.000 0.000 0.010 0.002 390 0.030 0.000 0.040 0.021 0.003 400 0.048 0.000 0.092 0.180 0.002 410 0.048 0.009 0.161 0.594 0.002 420 0.048 0.019 0.232 0.671 0.002 430 0.058 0.028 0.300 0.790 0.002 440 0.058 0.047 0.345 0.798 0.003 450 0.071 0.066 0.373 0.803 0.014 460 0.071 0.106 0.399 0.873 0.137 470 0.070 0.150 0.409 0.891 0.453 480 0.067 0.217 0.392 0.809 0.650 490 0.059 0.322 0.354 0.861 0.735 500 0.049 0.472 0.309 0.927 0.780 510 0.048 0.633 0.263 0.912 0.774 520 0.058 0.778 0.223 0.841 0.779 530 0.068 0.842 0.181 0.763 0.788 540 0.086 0.802 0.141 0.789 0.773 550 0.152 0.712 0.110 0.867 0.764 560 0.330 0.612 0.088 0.821 0.778 570 0.614 0.530 0.075 0.753 0.798 580 0.875 0.430 0.068 0.629 0.786 590 0.994 0.309 0.062 0.584 0.776 600 1.000 0.220 0.057 0.494 0.784 610 0.967 0.160 0.054 0.420 0.797 620 0.926 0.124 0.051 0.324 0.805 630 0.887 0.101 0.050 0.234 0.804 640 0.849 0.081 0.054 0.172 0.783 650 0.804 0.065 0.061 0.117 0.801 660 0.752 0.054 0.066 0.069 0.773 670 0.695 0.048 0.068 0.042 0.754 680 0.635 0.044 0.068 0.028 0.765 690 0.583 0.042 0.069 0.017 0.767 700 0.552 0.040 0.074 0.009 0.611 710 0.523 0.038 0.078 0.005 0.309 720 0.502 0.036 0.085 0.002 0.108 730 0.436 0.030 0.084 0.001 0.041 9