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Thread: Lumen levels and Human Perception

  1. #1

    Default Lumen levels and Human Perception

    I've heard multiple people says stuff like "You can't perceive a brightness change till 30% more lumens" or something like that.

    Is this just the word around the campfire? Or has someone actually taken pictures, ran tests... or shown this to be true in some quantitative fashion?

    I believe you guys, I'm just trying to explain to someone else.

    thanks!

    EDIT: Also, what % lumen change do you need before you get an easily noticeable or big difference?

  2. #2
    Flashaholic*
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    Default Re: Lumen levels and Human Perception

    Look up some info on the "db scale".
    3db change is considered to be the accepted difference that would be "noticeable change" by a wide segment of the population.
    It is somewhat a standard in the industry, such as seen in audio speaker specs like "20-20khz +/- 3db". That "+/- 3db" indicates that the speaker's response curve is going to be considered equally responsive to all frequencies within its boundaries to the level that most people would not be able to determine any audible inaccuracies in the levels, compared to the way they should be.
    It takes a doubling of the amplifier drive wattage to increase the sound level by 3db.

    The human eye also operates on the db scale, and the power requirements are similar to be able to increase the light ouput by 3db.

    It's logarithmic, and not linear.
    It doesn't mean that nobody can see any change at all, below the 3db threshold. It just means that many people won't be able to see a discernible change, but some sensitive people might notice some change. It's not going to be a huge change, in any case.

    However, it gets confusing when comparing lights of different outputs and different beam spreads. Then, you have other parameters entering into the comparison, and wider beams have to spread their lumens over wider areas, so they may not appear as bright as a light with a tight beam that concentrates all its light into a narrow focused area.
    This causes confusion to arise in people who try to compare two different lights with different characteristics, and then draw conclusions about the lights.
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  3. #3
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    Default Re: Lumen levels and Human Perception

    Another easy way to convince yourself of this is if you have an HDS Clicky with automatic step-down ( "burst" ) feature .... Try to see how easily you notice the approximately 20% decrease in output

    As to my own general rule(s), I find a 40% increase in output (all else being equal) to be "barely noticeable", a doubling to be relatively "easily noticeable" ... and a 10-fold increase usually seems to be a bit more than "twice-as-bright" (to me).
    Last edited by archimedes; 12-01-2012 at 03:37 PM.
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  4. #4
    Flashaholic*
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    Default Re: Lumen levels and Human Perception

    well, that depends on what you are looking for (bad pun). Caveat: humans perceive light on the dB scale, not a linear scale.

    CPF members that have the necessary intrumentation to test the lumen output as well as take some beamshots outside will certainly have the basis to provide quantitative as well as comparative statments. Look up some of the Selfbuit tests - very informative, especially the comparisons with other similar lights in the list.

    However, what kind of a beam does the light have? large spot? small spot? large spill? small spill? what is the transition between spot and spill? and then there is the human perception of color of the beam. Are you interested in a flooder or thower? Are you interested in the illumination in the center of the spot? Would you include the spill or not?

    examples: An XM-L LED in the usual relatively small light typically has a large spot with large spill - a wall of light at short ranges. The XP-G in a similarly powered light has a relatively small spot and "throws" farther. Meaning that the illumination in the center of the spot may well be "brighter" with the XP-G than with the XM-L even though the total output of the XM-L is clearly greater. ie: the Quark series, one with XM-L and one with XP-G.

    Since I do not have an integrating sphere, I use the ceiling bounce test to compare output levels, no numbers. I point the lights at the same spot on the standard house ceiling, away from walls, and look at the floor immediately under the point of aim on the ceiling. I then cover both lenses with my thumb or something relatively opaque and wait a bit for my eyes to adjust. I then begin alternating between lights by removing the blockage from one light output or the other. That is a poor flashaholics "integrating room". It makes no difference what the spot or spill size is, the comparison is reasonable, albeit not perfect. But no lumen numbers. For a correlation to lumen numbers, I could use use Selfbuilt's lumen test numbers as a basis, or even accept the statements from the manufacturer. However individual samples will vary.

    Short answer - it depends.

  5. #5

    Default Lumen levels and Human Perception

    I believe in the logarithmic perception thing (4x brightness ~ 2x perceived brightness)... it feels about right to my eyes. I can tell a 50% difference in lumens easy enough if from the same light switching back and forth between the modes, but it gets significantly harder to do so if from different lights (tint differences alone throws me off). Around a 30% lumen difference and they start seeming pretty much the same to me, and certainly, I would be unable to tell you what mode the light is on if only shown singly.

    Happily, I figured out a way to use a DSLR as an ambient lumen meter and can now objectively measure all my lights and modes. It's no dedicated lightbox, and only has a 1/3rd stop level of granularity, but I find it far, far more accurate than what I can see with my own eyes. For example, it has been able to show me exactly when my light steps down, which I have never been able to detect with the naked eye before.

  6. #6

    Default Re: Lumen levels and Human Perception

    Although the logarithmic relationship is not bad for generally describing how we perceived lumens, it is not so great over wide ranges. The reason for this is that human relative perceptions are complex, and involve a number of compensatory adaptations in both our sensory organ systems and our neural processing.

    As it turns out, modern psychometric research from the last fifty years points to a series of discrete power relationships for the relative perception of different classes of sensory signals. For a non-point source of light, a cube power relationship much better matches how we perceive relative to actual output.

    I have a detailed discussion of this - complete with academic references and visualization data from my own analyses - in this post.
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  7. #7
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    Default Re: Lumen levels and Human Perception

    Quote Originally Posted by reppans View Post
    I can tell a 50% difference in lumens easy enough if from the same light switching back and forth between the modes, but it gets significantly harder to do so if from different lights (tint differences alone throws me off).
    To borrow your quote to make my point:

    It is fairly easy to see the difference when switching brightness back-and-forth, sometimes even between different lights. We can see the change-over very well.
    But to really experience the "hardly can tell the brightness difference 30% up or down" is to have someone set a light to either X lumens or X+30% lumens and turn it on for you. When presented by itself, you will have a hard time saying for sure which setting it is.

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