DPSS RGB - White color balance question

Discussion in 'The Lounge' started by Godfrey, Sep 24, 2006.

  1. Godfrey

    Godfrey New Member

    Hello all,

    I will soon be purchasing materials to assemble and prototype a full color DPSS projector unit. The projector will contain three lab grade OEM analog modules, 473nm, 532nm and 635nm.

    I have heard on one forum that the ratio for the above wavelengths is 2:1:4. So, 300mW blue, 150mW green, 600mW red. I was told that with those specific wavelengths using that ratio I should be able to get a very nice white balanced beam. For 650nm modules the ratio changed to 2:1:6. For 671nm modules it was 2:1:8.

    I was wondering if this was fairly accurate. I did not want to design a projector which is heavy in one color as this will reduce the number of colors the projector is capable of producing. Ideally I'd like to get 16.8 million colors.

    As I understand it, analog modulated modules operate between 0v and 5v. To achieve 16.8 million colors - I would have to start out with a perfect white balance and each module would have to have 255 voltage levels between 0v and 5v.

    At this point, I am not sure if it is even possible. I do not know what the voltage increments are for analog modulation or if the ratio mentioned above is correct.

    I figured I would ask the experts.

    I have been searching the internet and a couple of forums, thought I'd ask here. I attempted to search on this forum but came up empty handed.

    Thanks in advance,

    Phil Godfrey
    St. Louis, MO
  2. Pangolin

    Pangolin Staff Member

    Hi Phil,

    The exact ratio depends on a number of factors, including the wavelength of the lasers used.

    Many people incorrectly look at the CIE Chromaticity diagram and try to deduce the correct ratio of colors using only that as a reference. But the CIE Chromaticity diagram does not provide the full picture with laser light because it does not account for the effects of Forward Scattering and Raleigh Scattering which tends to give shorter wavelength lasers a boost (i.e. blue lasers appear brighter than the diagram says that they should). I have seen RGB laser systems with a surprisingly low amount of blue power, and using a surprisingly short blue wavelength, give a surprisingly white appearance!

    Also, as counter-intuitive as it seems, 650nm or even 671nm red lasers are not half as bright or one-third as bright, as the CIE Chromaticity diagram would suggest. The rules for laser are a little different than this one table taken out of context suggests. Having worked with 635, 650 and 671nm lasers, I personally would rather have the nice beam profile and similar bandwidth and on/off behavior that a 671nm laser provides, than the large, crappy beam profile and inconsistent (with DPSS lasers) on/off characteristics of a 635nm laser (especially for graphics applications).

    In your case (and in most cases with commercial RGB lasers) the bigger question will be price and availability. When looking for lasers in the marketplace, chances are the green laser will always be the cheaper of the three anyway, and also coincidentally provide more power than you need. Similarly, 671nm red lasers are usually more available, at higher power levels and a lower price than 653nm lasers. For this reason, more often than not, RGB laser systems are not perfectly color-balanced, and software (such as Pangolin's LD2000 series) can reduce the amount of green for laser graphics applications, but still easily provide the full power of the laser when desired for beam applications. For beam applications, often, the degree of "white-ness" is of less importance than raw power.

    So, instead of trying to figure out precisely how much of which wavelength you need, I would first start with budget and availability, and work from there...

    Best regards,

    William Benner
  3. Godfrey

    Godfrey New Member


    Thank you for the quick reply. At one time I was trying to understand the chromaticity diagram but ended up with a headache...or eye ache. Anyway I was not basing the color balance on that model.

    As our eyes are very sensitive to green I knew I didn't need a high powered module for that wavelength. What I was trying to understand though is, in order to have a good balance I didn't want to have to reduce the output by whatever percentage for any one particular module. As I understand, if I were to have a green heavy unit - reducing it's output via the analog control signal I am therefore reducing the total number of colors that the unit is capable of producing. It makes sense. For example:

    We'll use a RGB color pallet - 255 (red) / 255 (green) / 255 (blue)

    All total, this should produce white and around 16,581,375 million colors.

    255 (red) / 128 (green) / 255 (blue)

    Now we have reduced the number of available colors to 8,323,200 - hmm, now that I look at that it isn't so bad. I was trying to manipulate the numbers in my head and apparently did not do such a great job of multiplying. I am not quite sure why I feel it is necessary to have the ENTIRE visible spectrum available.

    Obviously I can not rely on those numbers to translate into the number of possible colors the projector can produce.

    I agree with you on the beam profile of the 650nm and 671nm modules. The stacked diode design of most 635nm modules leaves you with a beam around 4mm +/- (x)mm. If you were to add optics to reduce the (I think stigmatism) you could be able to reduce the dot size. Now in regards to visibility of the 650nm and 671nm units, yes you would have to select a higher power unit.

    Costs involved. While I would like to keep them at a reasonable level, I am not overly concerned with the price differences of the red units.

    I have a few manufactures that would LOVE to send me their products for review, however at this time I am not prepared to do an evaluation...perhaps in a couple of months.

    Again, thank you replying to my post.

    Phil Godfrey