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LASER COLOUR BALANCE EXPLORED
By Carol Seidel

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.
Editors Note:  This paper was written some time ago and discusses colour balance using a PCAOM and a white-light laser.  The principals of photopic colour balance discussed here are equally applicable to today's solid-state RGB DPSS lasers.

ABSTRACT

The introduction of the Polychromatic Acousto-Optic Modulator (PCAOM) at the ILDA-ATW in New York in 1992 made it easy to bring an array of new colours to the laserist's palette. Where we had been satisfied with 7, 26 or 64 colours, we now had 16.7 million colours to choose from. Advanced laser graphics software even allows for the individual control of all of the six lines modulated by a PCAOM. This has led to an increasing interest among laserists in the issue of laser colour balance.

INTRODUCTION

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.
As aesthetic standards were raised, issues of colour range and balance began to take on more importance. Laserists began to express a need for better laser colour optimisation to realise the effects they were trying to produce.
The key to achieving aesthetic colour balance is designing for the highly non-linear response of the human eye to colour. The Human Photopic Sensitivity curve depicted in the graph (Figure 1) shows how colours of the same intensity appear to the human eye adapted for daylight.

Figure 1 - Human Photopic Sensitivity Curve

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.

Figure 2 - Laser power vs. wavelength

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.
Applying the colour sensitivity of the daylight adjusted eye as graphed in Figure 1 to the power distribution depicted in Figure 2, yields the apparent colour brightness graphed in Figure 3.

Figure 3 - Apparent Colour Brightness

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

The ideal solution for the laserist is to have the laser output tuned among the colours in a ratio which results in a balanced colour appearance to the human eye. To do this, the laser designer must "work backwards" from the aesthetically balanced colour brightness depicted in the graph (Figure 4).

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.
To take full advantage of the laser's brightness and colour capability, a ratio of R=4.6 : G=1 : B=2.6 is required for the mixed gas power distribution. A laser with this ratio could be regarded as a photopically (visually) colour balanced white light laser. The laserist would then have the full power of the laser available to create bright, colour saturated images of any colour.

SO . . .

Laser manufacturers need to address this issue to adequately meet the emerging demand for bright, accurate colour rendition for art, entertainment, advertising and educational laser displays.

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