Most PWM circuits run at a very high frequency, perhaps 50KHz. Putting a small bit of capacitance on the output would make it DC. Although I doubt that it would affect a meter, even without it.
Steve.
Well, it would not be very difficult for an engineer to develop a continuous-current regulator for a LED, the main problem is that its efficiency would be very lowi and it would heat much, much more than a PWM regulator.
Most PWM circuits run at a very high frequency, perhaps 50KHz. Putting a small bit of capacitance on the output would make it DC. Although I doubt that it would affect a meter, even without it.
Steve.
To my surprise the color temperature doesn't change as I dim it
Now what is a good design for a mixing chamber?
LEDs don't change color temperature.
And dimmable LED bulbs indeed shouldn't flicker. Although that's not an absolute; it depends on the driver circuitry. Some do flicker, usually at net frequency (60Hz where you live). Again, it all depends.
How about a cardboard box, painted white on the inside, with a piece of white cloth or white plexi? Or maybe a small softbox.
Not sure what the physical requirements are. Is this supposed to go over the Novatron? Maybe post a photo or two.
Ah, I see. Well, you could experiment a little with a box and two diffusion screens of some sort at a distance and the light source at a good distance from the first diffusor. While a double-diffusor setup comes at the cost of a considerable amount of lights, it tends to do a pretty good job at evening out any hot spots.
In terms of making the thing adjustable, I'm a bit hesitant when it comes to using a regular bulb on a dimmer, because the dimming behavior may not be as linear as you'd like it to be, and I'm not sure if the dimming range is sufficient. Again, it depends...just give it a try and see if it's good enough.
I was thinking of using 2 bulbs for good intensity.
I do not depend on the potentiometer position (or input voltage) to set it to certain level and thus linearity isn't a problem.
2 bulbs is one stop faster than a single bulb. A single bulb of twice the power achieves the same. Two bulbs of twice the power = +2 stops. Etc. Two bulbs should work fine, as should one bulb, or four or whatever takes your fancy!
OK, good.
Btw, my concern about dimming linearity not so much applies to the dimmer as such, but mostly how the LED fixture responds to it. But ignore this; if you're happy with the real-world performance, then that's all that's relevant.
Here's something that may or may not be appropriate; A vintage GE Footcandle selectometer
Light is behind a fairly large brass iris aperture that is controlled by a slide lever.
@ic-racer has built a pretty cool device. I've read through the thread about it.
Fill me in, how are you calibrating it? I have a "Master II" but did not think it could be calibrated.
I need a light source to test camera exposure meter as well as testing camera shutter speeds on auto.
I used to use a Beseler computerized 45 dichoic head for this. I just dial in equal filters to create neutral density.
I am thinking of a few ways.
1. Copying the dichroic head design using a mechanism that moves the filter in and out of the light path but instead of using filter I would use just an opaque gate to reduce the amount of light.
2. Inserting ND filter in the light path.
Dimming the light by reducing power would lower the color temperature.
Any ideas?
Right. You don’t. You just clean and balance the meter and then discover whether or not the assembly is calibrated.
This meter is determined to be calibrated for these conditions:
With a tungsten source adjusted in brightness as needed to make the remaining steps best meet the goal…
Aim source at a diffuse screen, adjust screen luminance by a combination of physical attenuation and adjusting source to screen distance until the Sekonic L-758DR indicates the nominal sample value, taking spotmeter readings off the diffuse panel.
Note readings of milliamps, voltage, and Weston Meter dial.
Samples were checked in third-stop increments from 1.6 to 50 candles per square inch on the exposure meter scale.
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