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Thread: Perceived brightness comparison?

  1. #1
    Join Date
    May 2020
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    11

    Default Perceived brightness comparison?

    I'm on the fence between a new controller for my existing 532nm 800mW DPSS system or selling it and going with a LaserCube 1W/2W RGB system...

    We mainly do outdoor, nighttime advertising of text for companies, so perceived brightness is the main factor for us.

    I found some specs online for the 1W/2W LaserCube and wondering what some experts think in terms of how the perceived brightness would compare to my existing 800mW:

    LaserCube 1W:
    Blue 445nm: 700mW
    Green 520nm: 100mW
    Red 637nm: 150mW

    Calculated this as around 225mW equivalent at 532nm - am I correct?

    LaserCube 2W:
    Blue 450nm: 1200mW
    Green 520nm: 400mW
    Red 638nm: 400mW

    Calculated this as around 630mW equivalent at 532nm - am I correct?

    Thank you in advance!

  2. #2
    Join Date
    Mar 2010
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    Raleigh, NC
    Posts
    2,146

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    I don't know how 520/532 compare in terms of brightness but I think that the red in that cube is a bit weak.

    Also, if you are presenting text you should be paying attention to the galvo specs. They are listed as >8deg @ 30000 but I don't know. Check with someone who has one to be sure and see how they do with text.
    LaserCube doesn't have an ILDA connector so you will be limited to some subset of DACs.

  3. #3
    Join Date
    Aug 2013
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    58

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    Brightness is a complicated question for several reasons:

    • Eye sensitivity as a function of wavelength depends on whether the eye is adapted for light conditions (photopic vision), dark conditions (scotopic vision) or somewhere in the middle (mesopic vision)
    • It depends on the spectral interaction of the laser with the surface on which you're projecting
    • It depends on contrast with the environment, both in terms of brightness and color
    • The mixing of monochromatic light sources (very sharp peaks in the spectrum) does not necessarily work the way that the mixing of broader band sources does
    • The brightness response of the human eye is nonlinear
    • The divergence of the laser will affect the irradiance/illuminance


    For very rough numbers, your 532nm 800mW laser is about 470 lumen under photopic conditions, and 440lm under scotopic conditions (it's not likely that you're dealing with scotopic conditions, though--if you can perceive color, that is a good indication that you're in the mesopic->photopic regime).

    If you add up the lasercube wavelengths, the 1W version gets you about 86lm (photopic) or 250lm (scotopic).
    The 2W version gets you about 276lm (photopic) or 578lm (scotopic).

    Again, the photopic numbers are probably more relevant here, and since you're talking about projecting on a surface you really have to know (or control) the projection surface to convert those raw (and very approximate) lumen numbers into any kind of real comparison. Essentially, what you're concerned with is luminous exitance which is the amount of flux (lumens) per unit area that are being reflected by the surface.

    In order to do that, you will need to:
    1. Use the divergence of the laser, the shape of the projected graphic, and the projection distance to obtain the total area illuminated by the laser and from there determine the illuminance (lumens incident on a surface per unit area)
    2. Adjust the illuminance as necessary to account for interaction with the reflectivity spectrum of the surface to obtain the luminous exitance (lumens emitted per unit area of the surface.


    Compare this with the luminous exitance of the overall surface from other light sources to determine the contrast of the laser projection versus the surroundings, which is really what you mean by "brightness" in this context. That all requires knowing something about the reflectivity spectrum of the surface.

    A final factor to consider is modulation response. DPSS greens, depending on quality, can have issues with stability that may reduce their brightness when modulated versus at a static power level. The degree to which this is a factor depends greatly on the initial quality of the DPSS system and what kind of life it's lived. DPSS greens also quite commonly drop dramatically in output as they get older, so by now any DPSS system still in use is quite likely running at substantially reduced power (again, dependent on initial quality and life). Is 800mW the nameplate spec, or have you measured it?

    So in short, it's a complicated question to answer quantitatively, but the first think I would do if you haven't already is confirm that your DPSS system is running at the power level you expect, otherwise any comparisons are potentially based on very faulty assumptions. Plus there is definitely value in having RGB as an option, for whatever that is worth to you or your customers--if you're projecting on a surface that absorbs a lot of green, then the RGB may well be vastly brighter!

  4. #4
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    While I agree that the question of apparent brightness is a complex one that has many variables that are difficult to control for, speaking in very general terms I believe it is safe to say that an 800 mw green-only projector at 532 nm is probably going to appear brighter than the 2 watt RGB Lasercube projector.

    How much brighter is open to interpretation; personally I don't think the difference will be a large one, but I do think it will be noticeable.

    To address Gary's point about 532 nm green compared to 520 nm green specifically; both the math and my own subjective experience show that 532 nm green is almost always going to look brighter than 520 nm green (assuming similar power levels and beam characteristics). This is due to the color sensitivity of the human eye, which peaks at 555 nm under normal (photopic) conditions.

    Adam
    Last edited by buffo; 06-03-2020 at 05:22.

  5. #5
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    Funny thing is to me 575 is brighter in daylight than 561. At night about the same. Maybe Iím broken.

  6. #6
    Join Date
    May 2020
    Posts
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    Quote Originally Posted by aberry View Post
    Brightness is a complicated question for several reasons:

    • Eye sensitivity as a function of wavelength depends on whether the eye is adapted for light conditions (photopic vision), dark conditions (scotopic vision) or somewhere in the middle (mesopic vision)
    • It depends on the spectral interaction of the laser with the surface on which you're projecting
    • It depends on contrast with the environment, both in terms of brightness and color
    • The mixing of monochromatic light sources (very sharp peaks in the spectrum) does not necessarily work the way that the mixing of broader band sources does
    • The brightness response of the human eye is nonlinear
    • The divergence of the laser will affect the irradiance/illuminance

    Wow! Thank you everyone for the detailed replies! I learned so much! I'm probably going to try and sell my existing 532nm system for now and then evaluate our needs going forward. Apparently, LaserCube is coming out with a green-only version and that might fit the bill in terms of perceived brightness while also having the easy to use LaserOS software.

    This post will come in handy for making these comparisons! Thanks again

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