[rollei_list] Re: CdS, SPD, and Selenium

----- Original Message ----- 
From: "Marc James Small" <msmall@xxxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Saturday, April 30, 2005 3:19 PM
Subject: [rollei_list] CdS, SPD, and Selenium


> At 09:28 AM 4/30/05 -0700, Peter K. wrote:
>>Just out of curiousity, why do you use Weston meters? 
>>Because tey are
>>batteryless? I used to use a Norwood Director, but hated 
>>it. I went to
>>digital with a Minolta IIIf years ago and never regretted 
>>it. Now I
>>use a Sekonic L508. Simple and easy and far more accurate 
>>then the
>>Director ever was.
>
> Peter
>
> I am not certain as to what you mean by "accurate" or 
> whehter your Norwood
> was out of whack.  Selenium more closely matches the 
> response patterns of
> modern emulsions than do either CdS or SPD or even Gallium 
> Arsenide.
> Selenium suffers from a weak response and a wide 
> acceptance angle but, with
> allowances for these quirks, it should provide a more 
> accurate estimate of
> the required exposure than do more "modern" light meters.
>
> Marc
>
> msmall@xxxxxxxxxxxx=20
> Cha robh b=E0s fir gun ghr=E0s fir!
>
> NEW FAX NUMBER:  +540-343-8505
>
   The acceptance angle depends on the design of the light 
gathering arrangement rather than on the type of sensor. Don 
Norwood's contribution was the hemispherical diffuser for 
incident readings. The standard accessories for the Director 
include a flat diffuser, which is for measuring the incident 
level of individual light sources and a perforated grid for 
narrow angle reflectance readings. The latter two are very 
useful for measuring lighting ratio and actual subject 
contrast (vs: lighting contrast). Norwood's original meter 
is the one made by Spectra and later continued as the 
Spectra Combi with an added low level photo sensor. The 
current Sekonic meter is almost unchanged from the original 
Director. If one uses it correctly it is quite accurate.
   The Weston meter has a multiple lens device over the 
photocell which controls its directivity. The idea was to 
include the entire picture area but Weston also designed the 
meter to make subject contrast readings. These are the 
reason for the extra markings on the scale. Originally, the 
readings for some types of subjects were supposed to be 
entered using the A or C pointer rather than the center 
arrow (which was marked B on the original meters).
   The spectral curves shown in _Photographic Materials and 
Processes_ indicate that "Silicon Blue" photoelectric cells 
have very similar spectral response to Selenium. Cds has 
greater red and less blue sensitivity. Without the built in 
filter of the Silicon Blue cell Silicon has a curve which 
rises with wavelength well into the infra-red.
   I've read an explanation of why films no longer carried a 
Tungsten speed rating after sometime in the mid-1960's. It 
attributes this to the change in meters from Selenium to Cds 
but the spectral curves make hash of this explanation.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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