Design of Apparatus for colour matching

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)

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

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 )

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

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

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

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

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

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)

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

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

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 300-600w

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

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

monochromator input by chance light source matches input of monochromator

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

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

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

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

Measuring light: X-Rite i1Pro spectrophotometer can be used as spectrometer 3.3nm accuracy (‘high resolution’ mode) produces 118 readings between 350-740nm very well suited to measuring displays cannot be calibrated might be considered a ‘toy’ by experts accuracy of 1nm is expected