Using the CA813 light meter to measure your front projector - Page 3

As far as the accuracy of the CA813 goes, I'm not sure it's as accurate as people think. I've done a lot of light measurements recently for contrast purposes and it deviates from a calibrated Progressive Labs C-5 colorimeter which has also been checked against another calibrated C-5 and a LightSpex. The CA813 and C-5 are close in the bright readings but deviate up to ~23% in the low luminance range. The CA813 also deviates from itself when one tracks the lux readings vs fc.

I've spoken to both Cliff Plavin at ProgressiveLabs and Wm about the CA-813 and both have said basically the same thing. The CA813 is a good value, but it's accuracy at the bottom end isn't that great. Wm went on to say that it uses two potentiometers for calibration purposes and pot's are notoriious for drift.

I agree. William and I had a conversation about it a long time ago. Of course Cliff has a PR650 sitting there so he's not ever going to pick up a lowly <$200 meter . I think, for relative measurments, using the same meter, it has good value, at higher readings.

I got one to more objectively test between different projectors. At higher levels, one could walk into a demo room, pop up a copy of my white pattern, use a yardstick to hold in the light beam (locate a known width at a spot that will have a higher reading than at a screen) and take a quick overall lumens output measurment.

When I was looking at lots of devices, it was hard to tell from site to site how much diffrence there really was in brightness. But for low level readings, I think it will only work well if there is enough light spill to take very "up-close" measurments that puts the meter out of it's lower end.

And I would never consider comparing their outputs aginst 2 different meters, except for the most casual ball-park numbers. Even then no so sure.

I figure a Minolta LS100 is about the only mortal priced meter that might be more reliable. But it ain't cheap either...

I like having one though, if for no other reason to monitor my own bulb. I hated that, "Is it really dimmer or just me" feeling.

Thanks!

Can we assume that the CA813 is fairly accurate at above, let's say 10lux ?

I got my CA813 yesterday and luckily found this thread to help me get the most out of it.

I'm looking at getting an upgrade to my Infocus IN76 and wanted to know where I stand for brightness with 1000 hours on the bulb. My quick check shows I'm getting about 544 lumens. Gives me a good idea what to expect if I upgrade to a Marantz VP15S1. As soon as I dig out the photography tripod I'll be doing some contrast measurements too.

Thanks to all for the good information!

Just found this after a lovingdvd post in the JVC RS20 thread.
Since the last post was in 2007, is this still the best value light meter to get now in 2009?

Thanks.

Mike

I'm with Mike and would also like to know if this is still the "recommended" meter for AVS'ers.

Don't forget this is a "budget" meter. I use one ... These are great for doing your own relative comparisons with your meter, but, I don't think so much for comparing your reading to mine. AFAIK, no other low end (priced) meter has come out. Not a big market for them. Next step up is still a Minlota (T-10; see below) AFAIK.

EDIT: I see I already posted the info I posted originally earlier in the thread, nevermind....removed..

I'm not sure if you meant the LS-x Minolta's (like the LS-100), but for meters that measure from the projector (illuminance meters) there is the Minolta T-10 for $940 that I have been told is the industry standard for that. The LS-100 is probably the industry standard for luminance meters (off the screen) for applications like this (the LS-110 goes brighter, but not as dark). The LS-100 looks like it is about $3300 new.

There might be an Extech or something in illuminance meters between the CA813 and the T-10. I agree with others that CA813s seem to vary for white measurements by a fair amount and I think people should also be on the lookout for the black level giving a reading other than zero even if pointed into a place with basically no visible light. In my experience they can have an offset were going from a dark place slowly to a brighter place the reading will go down until it reaches 0.00 and then start going up.

--Darin

Thanks Darin, Yes I did mean the T-10, thanks, I'll fix the post...

Unless Exech tech came out with somehting new, only one of their meters had a better claimed spec on accuracy. And when myself and William tried to pin them down on the numbers they basically said they were not going to give great results down low (marginal repeatablility and not very comparable between meters)

Best,
Scott

Thanks for the info.

I saw this post - link - and looked up info on CalMAN at SpectraCal.

CalMAN software - link.

X-Rite EyeOne Pro Spectroradiometer - link.
X-Rite i1Display 2 Colorimeter - link.

Prices - link.
CalMAN Standard with Eye-One Display2 Colorimeter - $299

Looked interesting.
Thoughts?

Down the rabbit hole...

Mike

I don't recommend tristimulus probes or the lower end spectraradiomaters for getting raw illuminance numbers. Better to use a dedicated meter, I think. But I'm not positive

The CA813 is a 8% f1' instrument. It is not any better than an EyeOne Pro at this except for sensitivity.

Mike:

You have the right person participating now. Jeff is a top calibrator and probably forgot more than I know about it. It was his input that drove me to switch from a different low end meter to the CA813.

Jeff, thanks for the input and help. By sensitivity, I presume you mean the CA813 would be a better at measuring low stimulus levels. Please correct me if that's not right. And as long as I'm asking, what do you think about the Minolta T-10?

Thanks again,
Scott

will the ca811 work?

How do higher end colorimeters such as "the Hubble" aka Sencore OTC-1000 perform as light meters? Is it still beter to get a relatively expensive dedicated meter?

I decided I better start with "baby steps" and just get a light meter before getting to fancy.

I was looking for prices and it looks like the Amazon price jumped from $165 -> $175 +S/H. Most other places are over $165.
I found this site - link - that is at $158 + S/H ($11.39 + 0.35 insurance in my case).

The PDF for that site had some nice tables and compared the CA811 vs. CA813.

Too me it seems that the only difference is:

CA811 - MAX function - "...highest value displayd every 400ms."
CA813 - PEAK function - "...highest single sample measured every 50ms."

...and CA813 "...provides a broader light response." Later noted in footnote as "Model CA813 offers higher sensitivity (200klux) and has a better spectral response to common light sources."

...and CA813 is slightly heavier

I'm not sure what that means.
I was just going to get the CA813 since that seems to be the AVS default (? ?), but any comments on the two meters above?
I placed my order.

Thanks.

Mike

 

ca811-813.pdf 239.6025390625k . file

Is the CA813 useful for setting gamma? My main tool is an i1pro, which is difficult to use quickly and accurately at 20 IRE and below. In order to set gamma in that range, is it reasonable to use the CA813 to measure at 100 IRE, convert the reading to ftL, do gamma calculations on the resulting measurement, and then set gamma at 20 IRE and below?

Does this make sense?
- Dave

No takers on this question? My assumption is that since gamma is a ratio of level relative to 100 IRE this shouldn't be a problem, but I'm wondering if the CA813 is accurate enough at 20 IRE and below.
- Dave

I've got some pretty good data on the AEMC CA813's accuracy.

Methodology
I measured from 10% to 100% stimulus patterns using a spectroradiometer (Orb Optronics SP-100). The display was a plasma. Next, I measured the same patterns using the CA813 by placing the reading head directly against the screen. By comparing the two I could determine whether the relationship between the two instruments remained constant or varied.

If it were constant, then a single comparison could be used to create an offset that would allow accurate measurements for all other levels using the CA813. This would also be strong evidence that the CA813's response was linear across a wide range of measurement.

Here are the results.

Level	10	20	30	40	50	60	70	80	90	100
Lux	1.14	5.95	14.8	27	42.6	65.9	92.9	124.6	161.6	203
Nits	0.62	3.25	8.21	14.99	23.58	36.54	51.42	69.18	90.05	112.8
fL	0.182	0.949	2.4	4.4	6.9	10.7	15	20.2	26.3	32.9
Conversion	1.82	1.83	1.8	1.8	1.81	1.8	1.81	1.8	1.79	1.8

As you can see, the CA813's response is very constant from approximately 1 lux to 200 lux.

One unusual aspect of this test is that I compared illuminance readings to luminance readings. However, once the offset between the two has been established, I see no problem with this.

From this data I am confident that the AEMC CA813 can be used within the measured range with confidence that its readings are accurate. Whether you could go below 1 lux and maintain that accuracy I cannot say. My reference instrument won't read any lower.

Edit: Although I cannot read lower using the reference spectro, I can use the Chroma 5 colorimeter to read considerably lower. I have also tested that against the spectro at higher luminance values and determined that its luminance readings are spot on. When I read the black level on the plasma using the Chroma 5 I get a consistent 0.06 cd/m2 or 0.018 fL. Using the CA813, I measure 0.11 lux. Divide that by the conversion value of 1.8 established above and you get 0.06, precisely the same luminance value obtained by measuring directly with the colorimeter.

From this I conclude that the AEMC CA813 is accurate down to at least 0.1 lux.

From this you can conclude *your* 813 is as accurate as *your* Orb Optronics SP-100

I know this is an old thread, but has some good information and just wanted to add one more tip.

For a black level reading if the meter is still reading high because it was getting a bunch of light and you want to speed things up you can just put your hand in front of the meter for a second to get it to drop quicker to the real light level and then let it settle out from there.

And I may have mentioned this earlier in which case this would be a repeat, but I think it makes sense to try to measure basically no light and see if the meter actually registers 0. The projector could be off or completely blocked for that. I've found that with one CA813 I can get a positive value measuring into a very dark corner and as I get the meter closer to places with more light I'll see the reading go down at first and then back up. I recall one time the darkest corner of my room was measuring something like 0.07 lux and that went down as I got it closer to more light. In that case I ended up adding .07 to my black level readings. Since then I've gotten a used Minolta T10 and I think that measures 0 in the darkest corner of my room with very little light (the room is mostly black velvet and I do this with all lights off and the projector on at least a blackout screen so I wouldn't expect even .01 lux there). Next time I measure I'll check what the meter measures with basically no light compared to the lowest non-zero value it can read just to make sure. The caps for the meters could be used for this too.

--Darin

Wouldn't a way to increase the dynamic range of the CA813 (to get CR info) be to take the 100 IRE measurement at the screen location, but take the 0 IRE measurement at a known short distance from the projector, and scale that reading down as the inverse square of the ratio of that distance to the screen distance? That should provide around a factor of 500 gain compared to measuring both at the distance of the screen.

Kevin

OK. I had assumed that 100 IRE would saturate the detector at close range, but I gather that isn't the case.

Kevin

If it does then you could just do something like measure 100 IRE and 20 IRE at the screen, then 20 IRE and 0 IRE (0 base) near the projector and multiply the 2 ratios for the ratio between 100 IRE and 0 IRE. Then there is no need to bring in more complicated mathematics like the inverse square law. The above works as long as the assumption that the ratios near the projector are the same as the ratios near the screen for full screen images is valid. Or put another way, that 20 IRE will drop off the same ratio between the closer measurement point and the further measurement point as 100 IRE does. This is effectively like measuring the drop-off between the 2 measurement points by using 20 IRE, so another way to calculate is to divide the 100 IRE measurement by the 0 IRE measurement and then multiply by how much larger the 20 IRE was at the closer point than the further point. For instance, if 20 IRE was 50x brighter at the closer point then 100 IRE should be too and so the 100 IRE measurement could be multiplied by 50 before dividing by the measurement for 0 IRE close to the projector to get 100 IRE / 0 IRE.

I hope that made sense.

With 2.2 gamma the above would increase the effective range the meter could measure about 35x (although that may be with some simplifying assumptions).

--Darin

Tom

I think you got lucky in the draw
Early on Jason started out using this meter and wondered why his numbers where different then others on this board and took slack for his reviews. I then bought the same meter based on his and others recommendation at the time. It was only then we found both meters delivered numbers far enough apart per same projector. We had no clue which was correct. At the time I did not have my spectro as reference. We both ended up giving our CA813 away. We now use the Minota T-10.