1. Design of Apparatus
for colour matching

2. Colour Matching colour matching is in principle very simple
one area of the screen is monochromatic (single wavelength) that can be set to any colour of the rainbow
second area of screen produced by the primaries (wavelengths at fixed locations)

3. Primaries conventional primaries are RGB
needs 3 lights
primaries for human visiual system are RGB+yellow
needs 4 lights
tunable primaries means adjusting the exact location of each primary

4. tunable primaries
tunable primaries implemented by pairs of fixed wavelength lights
the ratio of each pair is adjusted to achieve perceived wavelength
twice as many primaries are needed
8 primaries
573 1 . 2
Yellow
644 1 .
517 . 8
467
Red . 6
Green . 4
Blue . 2 . - .2 3 5 4 4 5 5 55 60 65
700 W a v e l e n g t h ( n m )

5. implementing tunable primaries Idea #1: mod medical grayscale LCD monitors
mod by removing backlight
design custom LED backlight with 8 types of LEDs
narrow wavelength LEDs at following wavelengths (nm): 450,475,500,525,560,590,620,645
divide screen into 2 areas:
monochromatic
color primaries
This approach has problems:
all pixels in each area will have the same colour
difficult to segregate 2 colour areas
there are no suitable high intensity yellow LEDs (560nm)
light is polarized
very poor light transmission

6. implementing tunable primaries Idea #2: use modified projector
mod the internal RGB filters to give 3 primaries/projector
can use inexpensive consumer projectors
each pixel can be controlled independently
Difficulties with this approach:
all use HID lamps which have very little cyan, green and red light
no optical alignment of multiple projectors

7. implementing tunable primaries Idea #2a: mod high end projector
some high end consumer projectors have xenon short-arc lamp
xenon short-arc has all colours at equal strength
precise control over each pixel
Difficulties with this approach:
image is not bright enough
less than 50 cd/m2
colour matching needs more than 300 cd/m2
cyan (500nm) and yellow (580nm) are missing

8. implementing tunable primaries Idea #2b: mod professional projector
most have xenon short-arc lamp
cyan and yellow can be achieved by blue+green and green+red
only 3 projectors needed
Difficulties with this approach:
very expensive
complex internal filter mod
projectors often very large
no lens shift for optical alignment

9. implementing tunable primaries Idea #2c: mod specialist dual lamp DLP projector
brute force solution … 1 projector needed for each primary
2x HID lamps
single DLP chip with color wheel
removing color wheel allows all wavelengths (grayscale image)
each projector produces image of one primary
external filter for each primary
superimpose images to form colour image
these are special dual lamp projectors, but which can run on one lamp, allowing a mod on 2nd lamp

10. internal single chip DLP projector mod
DLP projectors use a color wheel to separate light into RGB
remove color wheel and you get white only (black & white image)
insert custom (stationary) filter to get specific primary
small size replacement filter (15x15mm)

11. light pipe for DLP projectors
all light is focused onto a small area
easy to insert a small filter in place of color wheel
also possible to direct connect optical fiber to light pipe

12. simple external projector mod
place lens in front of projector lens
very simple mod
requires large filter (50x50mm)
large filters are expensive

13. lamp mod metal halide lamp is weak on cyan and red
mod second lamp to xenon short-arc
xenon short-arc is usually large
Cermax xenon short-arc is very small
but wrong reflector type – parabolic rather than ellipsoidal
small enough to mount inside ellipsoidal reflector designed for projector
simple but skilled mod
working cermax lamps from old projectors are very inexpensive and available from w

14. monochromatic light v.1 monochromator very inefficient
almost all light is lost (more than 95%)
difficult to achieve more than 100cd/m2 directly
much too weak to gathering this light to direct it into a projector
some issues with stray (unwanted) wavelengths emited
designed for 150w xenon short arc light source

15. Light Source xenon short-arc light source simulates natural sunlight
300w medical xenon short-arc light source x2
xenon short-arc light source
simulates natural sunlight

16. monochromator input
by chance light source matches input of monochromator

17. Monochromator in action
any wavelength between ultra-violet and deep red can be selected two monochromators allow two adjacent/overlapping stimuli to be produced

18. monochromatic light v.2 a DIY monochromator
using a linear variable filter to produce monochromatic light
about 50% efficient

19. LVF in optical fiber carries light to the LVF
light is directed back into an optical fiber on the other side
fibers are stable and LVF moves on linear stage
linear stage controlled by a stepper motor under computer control

20. LVF out
outgoing light directed by lens to optical fiber
Light path

21. Measuring light: X-Rite i1Pro spectrophotometer
can be used as spectrometer
3.3nm accuracy (‘high resolution’ mode)
produces 118 readings between nm
very well suited to measuring displays
cannot be calibrated
might be considered a ‘toy’ by experts
accuracy of 1nm is expected