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

[Ted Cousins]

Most battery-less photographic light-meters loose sensitivity over time as the sensor ages, especially Weston 'Master' Models up to model V.

A couple weeks ago, I found a Sekonic Model L-398A cheap on ebay and bought it because it still current and has an amorphous silicon sensor - much less susceptible aging than Selenium.

Pardon the pic quality:

I already owned a 4% accurate commercial light-meter ...

It has a sunken semi-dome filter ... I'm guessing for cosine response ...

About 14" from a cool white office desk lamp, the Sekonic 398A tells me 80 foot-candles (fc), 8Ev. The BTMETER tells me 39 fc.

Assuming that the l-398A hasn't been dropped, is the reading reasonable, I wonder?

Others:
My good-ish Japanese-made Weston Master6 tells me 26 fc. My original Sekonic 398 at 100 ASA with the dome on tells me about 65 fc, might be75 fc with the flat disk, couldn't be bothered to mount it.

 

[Doug Kerr]

Ted,

[The L-398A is my favorite photographic exposure meter.]

When you did the comparison using the L-398A, did you have the flat receptor on? This is the one prescribed for making incident illuminance measurements (not in the photographic context).

I note that we might expect the flat receptor to give nearly a cosine response, which is what is appropriate for illuminance measurement. The dome receptor used for photographic exposure metering purposes is often erroneously thought to give a cosine response, but in fact it gives roughly a "Norwood" response. The story behind that is fairly complicated, but I discuss it at length in an article on The Pumpkin. Here is a link to its listing:

The Pumpkin

Index page of Doug Kerr's The Pumpkin

dougkerr.net

However, there are often considerable discrepancies between the readings these meters give in the "illuminance" mode and what wholly non-phtographic meters (as typically used in illumination engineering and such) will show.

I never found our the reason for that.

The problem is discussed in Append1x B of my article, "The 'Norwood Director' family of photographic exposure meters". Here is a link to its listing on the index page of The Pumpkin:

The Pumpkin

Index page of Doug Kerr's The Pumpkin

dougkerr.net

Best regards,

Doug

 

[Doug Kerr]

Ted,

Measurement of illuminance

Keep in mind that the physical property illuminance describes the impact of light upon a surface of some certain orientation with respect to the direction of arrival of the light.

If we assume a small light course (perhaps a bulb at a considerable distance), then at some certain point away from the source the luminous flux density of the light "beam" will have some value. That is a property of the arriving light beam at that point, and does no presume anything for the light to land on. But the illuminance at that point must be considered as upon a surface of some certain orientation.

As a special case, if we imagine a surface to which the direction of arrival of the light is perpendicular, then the illuminance on that surface is numerically the same as the luminous flux density of the "beam" at that point.

But if the direction of arrival of the light beam is not perpendicular to the surface of interest (real or hypothetical), then the illuminance is the luminous flux density at that place times the cosine of the angle the direction of arrival of the light beam makes with the perpendicular to the surface.

Now if we have, falling on some surface, light coming from two different directions, the total luminance is the sum of the illuminance that each "beam" would create, taking into account for each the angle bwteen its direction of arrivaal and the perpendicularmto the ruafce.

That is why a meter to measure illuminance ideally has a "cosine" angular repose: so it will in effect make that reckoning as it goes.

But we must orient the meter so that its flat receptor (if that is what is has) is at the same orientation as that of the surface (real or hypothetical) of interest.

******

Norwood's dome - the short story

Phtographers early learned tbat when a subject (perhaps a film actor's head) received light from several directions, the choice of a desirable phtographic exposure was not obvious. Eventually, often empirical metering technaues were used that mesaured the light from differnt directions and combined those readings with some formula to lead to the "desirable exposure". Thisn was of course very tedious.

Don Norwood discovered that in many of the lighting situaions encountered in professional cinema work, an exposure meter would give a "desirable" exposure recommendation (that is what exposure meters do) with a single measurement if is was equipped with a hemispherical receptor.

Pressed by his technical colleagues to explain the theoretical basic for this, Norwood concocted a story that fails scrutiny in many ways. But the reality was nevertheless that, emporically, this type of meter was found to in many cases give a phtographic expsure recommendation that gave an exposre resuit considered to be "good" (and this is of course a subjective assessment).

A result is that very many photographic exposure meters, in their normal "phtographic exposure" mode, use a hemispherical receptor.

******

Best regards,

Doug

 

[Doug Kerr]

I note that much writing, luminous flux density and illuminance are considered to be the same thing, and are often indicated by the same symbol.

They are not the same thing. At a certain point distant from some light source of small angular size (as seen from that place):

• Luminous flux density is a measure of the "potency" of the beam of light itself at that place.

• Illuminance is a measure of the "impact" of that beam on a surface (real or hypothetical) at that place at some specific orientation with respect to the direction of arrival of the light beam.

Further confusion comes from the fact that the recommended symbol used for both those quantities is Ev (that "v" is a subscript, but I don't know how to do that on this text entry screen). "V" indicates "visible", and is meant to distinguish this photometric quantity, which is defined in terms of human visual response, from the parallel radiometric property, irradiance, which however is defined in purely physical terms (and for which the symbol is E).

It is easy to confuse this with the wholly unrelated symbol, Ev, from the APEX system, which describes in a logarithmic way a photographic exposure (combination of exposure time and aperture)

Adding to this mess is the fact that the luminance of a photographic subject (as for example determined by reflected light exposure metering) is often improperly given as a value of the APEX quantity Ev. (The notion behind that I will not trouble you with here.)

Best regards,

Doug