Battery-less Sekonic hand-held light-meter versus commercial (non-photographic) ...

[Doug Kerr]

Hi, Ted,

You say:

As to the constant 'C' that Sekonic states as 340 for the dome, I have read a lot that the common 'C' for a flat receptor is 250 ... haven't found what they say about that - you?

I had not been aware of that.

But I will say that perhaps 340 is typical for the dome (cardioid) collector (as we often see for Sekonic), and if the intimation of the ISO standard is followed (that C for a cosine collector should be 0.75 that for a cardioid collector), that would call for 255 for a cosine (flat) collector. So that fits what you have seen.

The mystery is that it looks as if for the flat receptor (used for duplex metering) and the dome receptor (used for "Norwood" metering) to give comparable results for common key:fill lighting setups (as we would hope), the value of C would have to be the same for both configurations.

So if the standard means to imply that the value of C should substantially differ between cosine and cardiod configuration, I do not understand why.

******
If we reverse engineer the exposure calculator for the Sekonic L-398A meter, and take the "illuminance" indication shown on the meter movement as "correct", that wouild essentially say that the value of C is very nearly 340, the "marked" value.

If this is a fact for the dome collector (cardiod directivity) configuration, then for the meter to exhibit a C of 250 in the flat collector (cosine directivity) configuration, with the flat collector in place, the illuminance indication on the meter movement would have be about 4/3 the actual value.

Doug

 

[Doug Kerr]

Ted,

Some very agricultural photometry done on my Sekonic L-398A exposure meter indicates as follows for a "head on" light source:

1. The indicated illuminance (as read directly on the meter movement) differed by only a factor of 0.84:1 between the dome and flat collectors (it being greater for the dome collector).

2. The indicated illuminance with the flat collector differed from the illuminance as measured by my illuminometer by a factor of 0.96:1 (the L-398A reading being the lesser).

Based on the ensuing photographic exposure recommendations, this implies a value of C for the dome mode of 337 and for the disk mode of 310. The "stated" value for this meter is 340.

So. based on that test data (in which I do not have a great deal of confidence), for this particular L-398A:

• The value of C is essentially the same for both dome (cardioid) and disk (cosine) modes, as we might expect logically.

• The values of C are essentially as stated for this meter.

Interesting.

Doug

 

[Doug Kerr]

Clearly, Sekonic (who we would certainly expect to know a great deal about exposure metering) did not get sucked in by the questionable intimation of ISO 2720-1974 that the value of C for the cosine directivity mode should differ from the value of C for the cardiod directivity mode.

It is not the only thing in that standard that should be ignored.

Best regards,

Doug

 

[Doug Kerr]

A complicating matter in trying to make precise analyses in this area is the difficult of precisely reading the "luminance" scale on the meter movement.

We see that scale (along with the exposure calculator) of our L-398A here:

The illuminance scale on the meter movement is marked with a logarithmic series of values, although the scale is not strictly logarithmic (nor linear either).

Those marks are for values separated by "one stop" intervals (20, 40, 80, etc.). There is an unlabeled secondary mark between each consecutive pair of major marks. This of course is the value that is halfway, on a logarithmic basis, between the adjacent major marks, a value of essentially 1.414 times the value of the major mark below it. So the mark between 40 and 80 is theoretically 56.57.

There are no finer markings. The boundaries of the white rectangles below the illuminate markings (these are part of a special usage mode I won't discuss here) are not intended to create at their ends minor scale marks; they in fact have different widths to each accommodate the size of the numbers in them.

But I do the best I can! Generally, I try and visually estimate the position of the needle to a precision of 1/4 of the distance between the consecutive major marks. (Those places are actually separated in value by about the ratio 1.19.)

Best regards,

Doug

 

[Doug Kerr]

Hi, Ted,

As to your A19 lamp. I have no idea what property "viewing angle" is.

Without an actual directivity curve for this lamp, we cannot make a reasonable estimate of the luminous intensity in a certain direction and thus the luminous flux density at some distance in that direction, which is the illuminance that would be caused by that light landing "head on" there on some real or hypothetical plane.

We could start by assuming that the lamp radiates approximately equally in all directions (except of course toward its base), and get a very rough estimate from that.

I would be glad to go through those calculations, but I am not sure that the result will be of much value.

Regarding that blurb, I note that "brightness" is not even a usable colloquialism for the property that is denominated in lumens, which is total luminous output. But we wouild need that property.

Best regards,

Doug

 

[Doug Kerr]

The solid angle embraced by "in all directions" is 4π radians.

If a light source, small enough that we can consider it to be a point source insofar as its effect at the distance of interest, has a total luminous output of Φ lumens, and it radiates that uniformly in all directions, then its luminous intensity in every direction is Φ/4π lumens/steradian.

At a distance, s (in meters) from the light source, the luminous flux density(amount of luminance flux crossing a unit area of a hypothetical plane at that distance, perpendicular to the direction from the light source, is (Φ/4π)/s^2.

The illuminance on an actual plane of that same orientation is that same value.

Best regards,

Doug