This document discusses color adjustment for an LED projection system. The goals are to calibrate, characterize and convert the LED colors. Experiments measure brightness stability over time, spectral radiance distribution, and establish a primary transform matrix. A forward and backward color model is developed relating digital counts to intensities. The chromaticity coordinates of D65 are identified. Testing with a colorchecker finds average color difference of 0.15 using the LUT model, and average of 1.31 without. In summary, the document analyzes the spectral performance, establishes a white point, and verifies color accuracy.

1. Color adjustment for LED
projection system
Color, Imaging and Illumination Center
Graduate Institute of Engineering
National Taiwan University of Science and
Technology 1

2. Motivation
 Color mixing can create a rich color changes.
 LED projection system is able to show the
correct brightness and color.
Goals
 Calibration, characterization, conversion.
 Analysis spectral, brightness and color.
 LED as a light source to be considered in the
CRI (Color rendering) and the relationship
between surface and color of projection.
Whitewashed walls
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3. Experimental design
Set up Experiments
• Projection system • Brightness stability time.
• Projection angle fixed at 15 • Spectral radiance distribution.
degrees elevation. • Primary transform matrix ＆
• 40cm from the surface to be inverse.
projected. • Electro-optical transfer
• Projection surface function.
• Assuming homogeneity • Find out D65.
refection of different • Analysis color differences.
measuring points.
• Range 30cm to 180cm on
the ground range.
3

4. Warm up ＆ Spectral radiance
distribution
200
Start to stabilize about 50 minutes
Tristimulus Values(cd/m^2)
195
190
185
Y(white)
180
175
170
0 25 50 75 100
Time (minutes)
0.02
0.018
Monochromatic spectrum in full compliance
0.016
with the overlay of white light spectrum.
Radiance(W/m^2*sr)
0.014
0.012
white
0.01
red
0.008
green
0.006
blue
0.004
0.002
0
380 405 430 455 480 505 530 555 580 605 630 655 680 705 730 755 780
Wavelength (nm) 4

5. Primary transform matrix ＆ inverse
Electro-optical transfer function
Digital count from 0 to 255 in increments of 8.Thus 33
levels of emission were measured for each of the three
channels. A total of 99 samples.
Forward model
1.0
0.8
Releative Intensity
0.6
Red
Green
Backward model 0.4
Blue
0.2
0.0
0 32 64 96 128 160 192 224 256
Code Value
5

6. Find out D65
1.0
0.8
0.6
color
0.4
gamut
0.2
0.0
0.0 0.2 0.4 0.6 0.8 1.0
D65 WHITE RED GREEN BLUE
x 0.3125 0.2544 0.6963 0.1916 0.1274
y 0.3300 0.2240 0.3032 0.7088 0.0768
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7. Error of LUT model
 Testing color: colorchecker patches
 Average : 0.1531
 Standard deviation : 0.0812
0.40
0.35
0.30
0.25
△Euv
0.20
0.15
0.10
0.05
0.00
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Colorchecker Patch ID
7

8. Verification of color difference
 Testing color: colorchecker patches
 Average : 1.3153
 Standard deviation : 0.7841
3.00
2.50
2.00
△Euv
1.50
1.00
0.50
0.00
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Colorchecker Patch ID
8

9. Discussion
 Color difference becomes large, and found that
the brightness value(Y) is the greatest impact.
Especially, the greater of difference in white .
 We found the tristimulus values ​ f D65 is even
o
larger than the original, confirmed that bright
light in the case of output, resulting in increased
chromatic aberration.
Summary
 Analysis spectral and Uniformity.
 Define the D65 .
 Verify the color difference.
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