Ray Flash ring flash adapter - luminance distribution
Note that this report does not bear on the matter of the "beam distribution" of the Ray Flash ring flash adapter.
Interest has been expressed in the uniformity of the luminance of the face of the Ray Flash ring flash adapter. (I will discuss the significance of this later.)
The following image gives a basic visual presentation of the distribution of luminance (actually, the luminance-time product, but since the time variation of the flash is consistent across the face, we can fall back and think only of relative luminance) across the face of a Ray Flash ring flash adapter mounted on a Canon Speedlite 580EX II flash unit, for emission in a direction almost parallel to the camera axis.
The image was taken with a Canon EOS 40D located about 20 feet from the flash unit. (Actually, the flash unit was on that same camera, and the shot was taken in a mirror 10' from the camera!)
Among other things, the shot gives a nice insight into the arrangement of the "light pipes" in the Ray Flash unit. Readily seen are the numerous little "facets" along the rear surface of the pipes that serve to direct part of the traveling light forward ("across" the pipe) in that region.
We see, in the upper sector of the ring (generally from about 10:30-1:30 "on the clock" a very limited luminance. Then, in the next "stripe" down (to about the "beltline" of the face, a greater luminance, with the remainder of the face exhibiting slightly less luminance, with fair uniformity.
Why luminance?
We rarely hear of the photometric property
luminance in connection with light sources, but rather more frequently in connection with illuminated surfaces (or self-luminous subjects) . But here I speak of the luminance of the source.
If our light source were a true point source (its cross-sectional size is inconsequential compared to the distance to an object upon which we are interested in the effect of the source), then we are intersted in its
luminous intensity-time product in the direction of interest, where
luminous intensity is the amount of luminous flux per unit solid angle (for a very tiny solid angle centered on the direction of interest). From here on, as a "shorthand" to save verbiage, we will not speak of the time factor, and speak only in terms of luminous intensity.
But here, we have an
extended source: one whose dimensions are substantial compared to the distance of interest.
In such a case, the effect on a distant subject (from any small but finite-sized "patch" on the source) is based on the "equivalent luminous intensity" of that patch. This is the product of the average luminance of the source within that patch and the area of the patch.
[We normally hear this relationship in the opposite direction: the luminance is the total luminous intensity over a very small patch divided by the area of that patch. Here we use it the other way up.]
Thus, to consider the "uniformity" of the "output" of an extended source (in a certain direction) across its face, we must consider the distribution of its luminance (in that direction) across the face. So I speak of that.
Why do we care?
Well, suppose that our ring light had a drastic variation in luminance around its face. Then a centered circular object would not exhibit a "uniform" shadow around its periphery. (Note that non-centered objects will not in any case exhibit a uniform shadow.)
What about beam distribution
We heard it said (not by
moi) that a Ray Flash installation "exhibited a narrow beam" which could result in vignetting. How is that related to what I report here?
Hardly at all. That property relates to how the luminance of the face of the flash adapter (for any "patch" of interest) varies with angle. (Recall that the image I showed demonstrates the luminance along the axis of the system.)
A different test is needed to gain insight into that property, one I have not done (even designed) yet. I hope to deal with that in a time frame less than that required to get the first reaction to my initial report on this unit.
Best regards,
Doug