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COLOUR BALANCE EXPLORED
Note: The article below first appeared in the Winter
1994-95 edition of Laser Effects: The Light Show Quarterly and is reproduced
here by permission of the author.
The introduction of the PCAOM for laser colour modulation brought ease of
use and simplicity to colour laser projectors. The resulting availability of
full colour to many more artists and laser display producers lifted laser
graphics to new standards of colour rendition and artistic expression.
WHAT DO WE HAVE NOW ?
The commercially available sources of multicoloured laser light are predominantly argon/krypton (mixed gas or white-light) lasers with a power distribution among the colours along the lines of the graph (Figure 2). This colour/power distribution (if it can be maintained over a reasonable operating lifetime) has come to be regarded as the norm for white-light lasers. It should be noted in particular that a manufacturing goal of between 25% and 30% red power on initial delivery has been considered acceptable for this purpose in white-light lasers. Based on the graph, this would seem to be satisfactory performance.
The human eye does not, however, see colour according to
this graph (Figure 2). The eye is much more sensitive to the yellow/green
region of the spectrum than the red or blue. As shown in the Human Photopic
Sensitivity graph (Figure 1) the krypton line at 647 nm is almost five times
more difficult to see than the Argon line at 515 nm. argon blue at 488 nm is
over 2.5 times more difficult to see than the green.
The resulting effect is images that appear overly green or deficient in the red. The laserist can compensate for this to some extent by discarding the excess green and blue . This method of balancing the colour brightness of course results in significantly reduced overall brightness, effectively limited by the photopically weak red output.
LET'S GO WISHING
Figure 4 - Aesthetically balanced colour brightness
Compensating for the colour sensitivity of the eye yields an ideal laser power distribution shown in this graph (Figure 5).
Figure 5 - Ideal Laser Power Distribution
The photopically balanced "ideal" RGB power
distribution resulting from this calculation reveals a surprising result: To
look equally bright to the human eye, the red output must approach 60% of
the total RGB power.
SO . . .
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