Light Meter Spectral Response

I suspect the older zoom meter I'm using is overly
responsive to green. Is it quite usuall for meters to
favor some color more than others? Dan

sure, response curves vary a lot, a peak in the green region is considered
desirable because the human eye responds strongly to yellow-green lighting
(shades of our arboreal ancestors? ;-0)

see notes at http://medfmt.8k.com/mf/meters.html etc.

hth bobm
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* Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 *
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Bob Monaghan wrote:

[...] a peak in the green region is considered
desirable because the human eye responds strongly to yellow-green lighting
(shades of our arboreal ancestors? ;-0)

If film's (the thing in the chain between scene and human eye) spectral
sensitivity mimics our eye's response, then it's desirable that meters do
the same.
And that may or may not always be the case.

Looking at some randomly chosen film spectral response charts, i get the
distinct impression that manufacturers aim to produce films that have a flat
response curve. If so (and i always understood that it indeed is so) it
would be desirable if meter's response does not (!) mimic our atavistic
eyes.

So my answer would be the opposite: "a peak in the green region is
considered [NOT to be] desirable"

Q.G. de Bakker wrote:

Bob Monaghan wrote:


[...] a peak in the green region is considered
desirable because the human eye responds strongly to yellow-green lighting
(shades of our arboreal ancestors? ;-0)


If film's (the thing in the chain between scene and human eye) spectral
sensitivity mimics our eye's response, then it's desirable that meters do
the same. And that may or may not always be the case.

So my answer would be the opposite: "a peak in the green region is
considered [NOT to be] desirable"

First of all, I think film should not mimic eyes spectral responce - if
we want to replicate reality. Film should have 100% flat responce as we
don't want to multiply the eye's responce curve - we want to apply it
only once in the eye. We want to look the film/slide/picture as we
looked at the scene.

This leads to the same conclusion as you: meter should also not have any
peaks but have the exact same responce as the film.

But then again: this rarely affects our photographs in any significant way.

Severi S.

Q.G. de Bakker wrote:
So my answer would be the opposite: "a peak in the green region is
considered [NOT to be] desirable"

But digital sensors have twice has many green sites as
red or blue (except foveon). So maybe there is an advantage ?

"Bob Monaghan" wrote in message
...

sure, response curves vary a lot, a peak in the green region is considered
desirable because the human eye responds strongly to yellow-green lighting
(shades of our arboreal ancestors? ;-0)

Uh, that's backwards. A meter that is oversensitive to green would give
inflated readings, thus less exposure, thus dull results.

Photographic light meters are supposed to accomdate the color sensitivities
of the film. OTOH, light meters intended to evaluate light in terms of the
human eye have different standards (check with the standards groups.)

Now to the issue - while the human eye is, indeed, most 'sensitive' to light
in the green/yellow boundary area (540nm) and interprets is as rather
bright, we all know that most B&W images metered incidently make greens
rather dull to our eyes; add to that the fact that some B&W films _are_ less
sensitive to green and it seems highly unlikely that our photographic meters
are calibrated to the human eye.

Roger I. McMillan wrote:

But digital sensors have twice has many green sites as
red or blue (except foveon). So maybe there is an advantage ?

Only perhaps if they change colours such that they look twice as green too.

"Q.G. de Bakker" wrote in message
i.nl...
Roger I. McMillan wrote:

But digital sensors have twice has many green sites as
red or blue (except foveon). So maybe there is an advantage ?

Only perhaps if they change colours such that they look twice as green
too.

Us B&W people find the green channel to be the cleanest. Can someone
explain that?

"Q.G. de Bakker" wrote in message ...

Looking at some randomly chosen film spectral response charts, i get the
distinct impression that manufacturers aim to produce films that have a flat
response curve.

I am not sure if this is true. A flat response curve would mean that
in case you take a photo of a colour spectrum image (with a range from
violet to red) that you would see a neutral grey all over, and no
differences between the grey scale values of colours. But it is pretty
obvious that green and yellow appear brighter to the human eye than
blue and red.

In color metrology there is a more or less standard chart of human
eye's response to colours. Film manufacturers try to imitate this
'curve' to get the same impression on film which the human eye sees.
This was not easy since silver halogenides are sensitive in the blue
range only, and light absorbing colorants had to be added to reach the
goal of panchromatic (which means all-color) films. But still today
most b/w films are more sensitive to blue - the blue sky usually shows
much brighter on the film than it appears to the human eye.

Selenium meters had a response curve which was rather close to the
human eye curve. CdS meters are somewhat different. It's difficult to
obtain the same curve with silicon photodiodes, that's why most
manufactures use SBC cells (silicon blue filtered cells). They add a
blue filter to dampen the red sensitivity and to increase the
sensitivity in the blue and green range.

However, the only reliable way to compare meters is to use a neutral
grey card which eleminates any problems with color response.

Winfried

Winfried Buechsenschuetz wrote:

Looking at some randomly chosen film spectral response charts, i get the
distinct impression that manufacturers aim to produce films that have a
flat
response curve.

I am not sure if this is true. A flat response curve would mean that
in case you take a photo of a colour spectrum image (with a range from
violet to red) that you would see a neutral grey all over, and no
differences between the grey scale values of colours. But it is pretty
obvious that green and yellow appear brighter to the human eye than
blue and red.

True. That's why they have invented filters. Right?

If a film is presented with an "equi-energy" spectrum, the correct respons
would indeed be a "flat" image.
In a typical real life scene, different colours in the subject filter out
different amounts of light from the spectrum in which all wavelengths films
are sensitive to are prresent in more or less equal amounts. That's how
different grey tones in B&W photography are produced. Not by selective
sensitivity.

[...]
However, the only reliable way to compare meters is to use a neutral
grey card which eleminates any problems with color response.

Not so. If a meter has a low sensitivity for any part of the spectrum, that
pat wil not be present in the result it presents. And that reradless of
whether light in that part of the spectrum was or was not present in the
light int was measuring. Errors are then pre-programmed.

Don't think that using a neutral grey card makes an difference. All it does
is present a non-selectively reflecting surface (unlike coloured subjects,
which by being selective in what they reflect produces diferent grey levels
for different colours ;-)).