Does anyone know about the methods used by flash meters?

I know and understand about incident light meters, bu how does a flash meter work with such low duration, high intensity light flashes and calculate exposure/aperture settings?

Any info would be appreciated, as would a pointer to a representative schematic.

 

You probably have to integrate the light output (from a photo transistor or diode) and hold it in a sample & hold circuit. I would think by integrating it you will get both the intensity and the duration combined as one voltage, representing the total amount of light . And when correctly multiplied with the cameras f stop and ISO setting, then you will have your exposure. The exact figures will of cause have to be worked out experimentally.
Edit:
You should be able to use an old light meter as the bases for your readings.
Be careful, these meters are in the uA range and will be destroyed by the raw (full) output from an integrated circuit.

 

Can a sample and hold circuit trap such a small pulse long enough to yield an acurate reading though?

What about using a microcontroller and photodiode to catch the peak amplitude and duration, you could add all sorts of functions such as rise and fall time, there again the sample rate needs to be very high so it might no be possible.

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I don't think the duration of the flash is an issue. The sequence of events is as follows:
The camera will be set to its flash syncronisation setting. This sets a shutter speed of either, 1/60, 1/125 or 1/250 of a second. (I believe Nikon can use 1/400 but I'm open to correction).
When triggered, the focal plane shutter curtain opens completely.
The flash is fired
The shutter closes.

In order to achieve correct exposure, the camera's aperture setting is the critical setting.

A flash meter, I reckon, should, ideally, be able to trigger the flash setup itself, and monitor the light level received during the camera synch time. I would think that a PIC could work here (I have no idea about programming though!) and would simulate the camera operation to "open" the shutter, fire the flash, "close" the timing period and calculate the amount of light received and display results on a LCD display.
As to how to design and program something like this - I am at a loss, which is why I'm here!

 

The camera's "sync time" has nothing to do with the the amount of light received from the flash, it only effect the exposure as far as the the ambient light is concerned.
This is because the flash duration is only a small fraction of the cameras "sync time".

Both meters work essentially the same whether they are standard meters or flash meters, except that flash meters have a much higher response time and holds the reading. At last that is the way I understand it, I have newer owned a flash meter.
Of cause all this is even more complicated with cameras that use the TTL & EX type slaves. It is much easier to use the camera as a "light meter" since the "film" is essentially free and you have virtually instant feedback.

 

Hi Rolf.
I agree, but when doing flas photography, th shutter spped is fixed to a particular speed to ensure that the hutter is fully open when the flash fires.

Exposure control is therfore reliant on aperture setting to regulate and cater for the light level. This is where a flash meter becomes useful. While modern cameras have some excellent flash algorithms built-in, they can become a tad confused when several flashes of differing intensities are used.

 

This is news to me, have never heard of it. Please elaborate.

 

Funnily enough, I have one (somewhere!) - although I've never actually used it. I bought it back in my caving days, when we were planning some underground photography in some pretty large chambers, using multiple flash lights.

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Hi Rolf.
I'll rply in detail tomorrow some time, but, re-reading my post, it's a tad ambiguous as I failed to specify that there are camera metering problems with TTL systems (imho) when using multiple light setup.

 

Since you refaced your statement, I full agree.

It is why you can use a slave flash trigger like mine http://www.pbase.com/sinoline/slave_flash_trigger_iii that triggers on the last flash. But you have to modify the light output of the camera flash or the slave, maybe both in order to get the correct exposure. I don't think there are any other practical way of doing it.

 

Hi Rolf. Quite right - hence the need for a flash meter. I see that some incident light meters can be used by puting a diffuser dome over the sensor. I would imagine that there is some sort of sample and hold circuitry involved to display the reading.

As this is for a home studio - conformity to commercial or professional standards would not be an issue. As long as I get consistent readings, then a scale of aperture versus light output would be quite adequate for my use.

Of course, if it COULD be correctly calibrated, that would be a bonus.

I intend to try your system, it sound incredible! Well done.