Zalco ZLM1 meter

I have re-discovered a meter that I bought several years ago from the US manufactuer

in Oregon (I think). I believe it cost about $200 and is both reflected and incident and

uses a Si cell. It is still functioning well. I have checked both Google and Yahoo and

can find no reference for this meter or the company Zalco Inc. Does anyone know

what happened to the Zalco Inc company?

I bought one those meters years ago and yes it was from Oregon. I found it to be the most user-friendly meter and still prefer to any other. I don't know where Zalco went but if you find out, please let me know. Mine still works too. I would like to find another used condition one too. Greg Email: gkatz@ptd.net

I had access to several photo diodes in 1972. They were very small barrels -- gold plated. They had been used to read 9 hole paper punch tape, a very early memory media for teletype machines.

 

Anyway, I had read about a good meter that used a cadmium sulfide cell in Radio - Electronics magazine. This meter showed how to get impressive dynamic range.

 

I decided that I could get a very linear response with the new photodiode, so I built a circuit to test out the concept. The meter had excellent linearity, and dynamic range, but did not do well over a variety of light spectra, due to the very narrow response of the silicon cell. The meter was off about one stop for incandescent light, and three stops for fluorescent light.

 

This idea was shelved until about 1980, when Gossen introduced their meter that featured a blue enhanced photo cell. I reasoned that the new diode could be made to solve my earlier concerns, and was able to gather the necessary people to build the meter. I quickly found out from the prototype that the blue cell still needed additional filtration to function properly. As far as I know, only the ZLM1, and perhaps some Zone VI enhanced meters have more than one filter for spectral matching.

 

I'm not sure about this, but the idea of using a silicon diode for photographic applications could have been Zalco's original thought. If so, the patent could easily have fetched a lot of money, since the idea is used in virtually film and digital camera for exposure control. My boss in 1972 did not show any great interest in the project. At the time, it was the date of concept, and not the date of filing that constituted legal ownership.

 

Your meter is still working due to careful material selection. ZLM1's movement is a military grade taut band movement, whereas Gossen's is a D'Arsonvaal jewel suspended type. The wind is a high grade quartz, very difficult to cut.

 

Jim Bausch, president, Zalco, Inc. (An Oregon Corporation)<div></div>

I bought my ZLM1 new about 30 years ago. About 20 years ago I bought new the Sekonic L-508. I gave my ZLM1 to my son who still uses it for video. I missed the ZLM1 and was browsing Ebay the other day and found a ZLM1 for $5. Well I won it for $5 plus $5 shipping. It arrived today and it functions like new. Sometimes you get lucky!

I still have some prototype silicon diodes produced in the research dept. of the company where I worked in 1975. Some were fitted in bought-in TO-18 windowed 'top-hats' with a cyan filter to reduce the red sensitivity. So Si diodes with a modified response were obviously reasonably common by that time.

 

In around 1980 I built a home-made flash/ambient light meter using a so-called 'eye response' silicon photodiode that was readily commercially available.

 

The linear response of photodiodes actually presents a design problem, since we need the exposure indicated in stops or EV steps. I.e Log2(Exposure). My solution was to use a logarithmic amplifier to drive the analogue MC meter, although using the voltage, rather than current, output of the diode would partly solve the problem. However, neither a log amplifier, nor using the diode in voltage mode, overcome the temperature dependence.

 

I never did solve that properly, and had to manually zero the meter before every use.

 

Of course, the advent of cheap programmable PIC chips makes such issues a thing of the past. With no need for delicate analogue meter readouts, chopper-stabilised amps and the like. It feels like it was a different century..... oh wait. It was!

@rodeo_joe1Hi Joe. Not sure this message can get to you, but I would like to comment about the design of my ZLM1 exposure meter. This meter uses a log amp as does your design.  It has a linear response to the log of light intensity. The temperature characteristic of the log property is corrected by use of a thermistor. This meter was introduced in 1983. The last one was sold about 1994. Jim Bausch ~ Zalco incorporated.