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Old 10-23-2010, 08:49 PM #1
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Lightbulb Using Hoya R-72 IR transmission filter to measure IR from DPSS lasers [videos]

Introduction

Cleaning house a few weeks ago I ran across my Hoya R-72 infrared transmission filter. These filters are used for infrared photography. They are designed to block all visible light below long red wavelengths of approximately 720 nM. As such they are perfect for measuring infrared emissions from lasers. Because most digital cameras have infrared blocking filters, long exposures need to be used; any transmitted visible light will ruin the infrared picture. So the percent of visible light blocked is very high. The spec sheet for the filter can be found here:

Infrared Transmitting Filters from HOYA OPTICS: Leading supplier of advanced optical materials & components

You will note from the first graph on the above link that the R-72 blocks very close to 100% of visible light below 700 nM but transmits around 91% of infrared radiation above 790 nM or so. Since many of us are interested in the IR emissions of 532 nM lasers, which when unfiltered leak infrared radiation at around 808 and 1064 nM, these filters can be used to block the 532 and thus allow for a direct measurement of infrared emissions of our lasers. Because the filter is only allowing approximately 91% of IR through, the measurements should be multiplied by approximately 1.1 to give the actual IR emissions of lasers. The IR stated in the text below reflects this adjustment.

Also, the glass on the Hoya R-72 is very thick and so the filter itself has decent heat capacity. The IR filtering is not merely provided by a surface coating, but rather by the material of the glass itself.

The filters can be purchased online for as little as $50, for example here:

HOYA 55mm Infrared IR (R72) High Quality Filter New - eBay (item 360282427878 end time Nov-15-10 22:00:42 PST)

The filters come in different sizes, the smaller diameter filters tending to cost less. (I have a 67 mm filter, which fit on one of my old Canon lenses (now sold) but 55 mm filters will work just as well and are cheaper.) There are various other cheaper IR transmission filters on the market, but I don't know anything about their quality. Hoya has an excellent reputation and is used by professional photographers.

Videos

The first five videos below demonstrate measurements of IR from a few green DPSS lasers, some with filters, and some without. The remaining videos show the blocking ability of the filter at other wavelengths, including a stress test with the filter blocking one of the very nice 1W 445 lasers made by yobresal. Spoiler: the filter blocks all the energy; less than 1 mW is transmitted!

I used one of the excellent Ophir head based laser power meters sold by forum member Nospin.

The first video is of my monster overspec Optotronics RPL-260 (Thanks Jack!) Optotronics claims their filter blocks 96% of infrared; I believe it from the video. The unit typically puts out around 8-9 mW of IR, while the laser is outputting from 450-508 mW of power. (Yes, I did say monster overspec!) The first couple of minutes of the laser are not included as the laser needs a few minutes to warm up to operating temperature.



The second video is of the Budget Gadgets "Focusable Green Beam Laser Pointer Pen Flashlight AG-1006 ": $33.99 Focusable Green Beam Laser Pointer Pen Flashlight AG-1006 50mW/100mW/150mW/200mW - BudgetGadgets.com This laser is definitely not filtered, putting out around 70-71 mW of IR out of 200 mW of total power. That means that the actual power of the 532 nM emitted by the Budget Gadgets laser is around 130 mW. Since the laser only costs $114, all in all, the laser was not a bad deal, if you don't mind the infrared. The laser still burns like 200 mW, but is only has the visibility of a 130 mW 532 nM laser. With it's long duty cycle, and ability to focus, it is still a good deal in my opinion. Again, I allowed the laser to warm up a before starting the video.



The third video, NovaNoFilter, is an aprox 60 sec. run of a Novalasers X-100 unit with no filter (Edit: it still has its internal filter, but the Hoya is not being used). Note the peak power of 131mw. The documentation provided with the unit gives its peak power as 136mw. So on this run the peak was very close. The documentation lists it's average power as 115.3mw, again close to what this run shows, except possibly near the end. Also, the duty cycle for the Nova unit is given as 90 seconds, but I ran for just a bit over 60 here. One problem with the Nova unit is when it has warmed up, it starts mode hopping like crazy and suffers dramatic loss of power. You will see that in the next video.



The fourth NovaFiltered is a 60 sec. run of the Novalasers unit with the Hoya IR filter. Note the complete attenuation of detectable IR energy when the IR filter is used. Novalasers claims the filter in their unit blocks 99.5% of IR. I believe it from this video. Less than one mW IR is reaching the sensor. Very dramatic! At the end when the filter is removed, note the low power of the unit. The beam has gone to crap at this point, TEM 01, and the laser is putting out sub-spec power. This unit has been doing that since I got it. I have claimed the warranty on the unit and am awaiting replacement. Nevertheless I can't fault their IR filter.



The fifth TriAngleLights is a very long run on the Peltier and fan cooled 150 mW TriAngle Lights 532nm lab unit (TTL version). The TriAngle lights laser is very well-known to have no IR filter. It is a very stable laser and its IR emissions are stable as well, staying around 71mw. During this run I also moved the filter around to demonstrate the uniformity of the blocking and transmission across the surface of the Hoya filter.



The sixth 445FilterNoFilter is a short run of one of yobresal's 1.1-1.2 W 445nm lasers showing the ability of the filter to block high power. Note how after inserting the filter, the power drops to 0, then briefly recovers to 4mw or so. I interpret this as being due to localized heating of the filter by the high energy of the laser and the filter re-radiating the heat as IR, which the LPM picks up. But then after the heat starts spreading through the glass, the LPM sensor adjusts, detecting it as ambient heat, and reports the correct value of zero transmitted IR. The ability of the Ophir head to adjust to changing environmental conditions is discussed in threads on LPF about this LPM. I think it is a very nice LPM feature. This test shows that the R-72 is blocking over 99.9% of 445 nM light.



The seventh video, 650nm, shows the total blocking of the filter at the 650nm wavelength, which is getting closer to the transmission range for the filter. The laser used here is one of the O-like 200 mW lasers. The battery is somewhat low and the laser is not putting out 200 mW in the video, although I have measured it near that before.



Conclusion

Most measurements of IR I have seen on this forum rely on using filters that block IR. With these filters, there are two things that have to be taken into account: the attenuation of the visible laser light as well as the percent of blocked IR. Because the Hoya filter blocks pretty near 100% of visible light in the wavelengths of interest, one only has to deal with the IR transmission percentage, which Hoya documents carefully. Thus this method of measuring IR seems superior to the other method.

One unknown in the above is the transmission of the Hoya at 1064 nM. The transmission graph of the R-72 stops at around 850 nM, but the trend on the graph is toward greater transmission at higher wavelengths. I plan to contact Hoya and ask about the filter's transmission at 1064 and update this post accordingly. They do state that the filter transmits > 85% of IR above 720 +- 10 nM, so this suggests that the trend on the graph extends to 1064 nM.

If you have already invested in an LPM, I think that the small extra cost of obtaining an IR transmission filter is well worth it for getting accurate measurements of IR emissions of DPSS lasers (not just 532 nM). This is especially true for laser sellers. Perhaps other users can compare different transmission filters to find a lower cost alternative to the Hoya R-72. Note Hoya sells other IR transmission filter beside the R-72, but I have not priced them.



Last edited by Analogue; 10-23-2010 at 09:33 PM. Reason: Clarification.
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Old 10-23-2010, 09:31 PM #2
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Default Re: Using Hoya R-72 IR transmission filter to measure IR from DPSS lasers [videos]

Analogue ---

A big +1 for you. I was hoping someone would take the time to show how the lenses need to be "calibrated" for absorbsion, reflection etc. I hope everyone reads this.
My IR absorbing lens reflects almost 15% of the green -- I measured the reflection!!!
This energy has to be accounted for in calculations.

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Old 10-23-2010, 10:15 PM #3
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Default Re: Using Hoya R-72 IR transmission filter to measure IR from DPSS lasers [videos]

Thank you Mike.

The only IR absorbing lens I have played with was from an old point-and-shoot digital I took apart-- and it had horrible green transmission-- I recall it transmitting something like 30% of green. (I originally toyed with the idea of using it on one of my TriAngle Lights labbies, but abandoned the idea when I saw how poorly it performed. Do you know what the green absorption is on your lens?

I'm actually in the market for a good deal on a high quality IR absorbing lens, but I haven't really started looking yet.
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Old 10-24-2010, 12:13 AM #4
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Default Re: Using Hoya R-72 IR transmission filter to measure IR from DPSS lasers [videos]

Analogue ---

I have a couple filters and forget now the correction factors. I have an 808 pen and other greens to pull back the corrections.
I recently saw a review where an IR filter dropped the green total power by a bunch and no comment was made about calibration. The bunch missing was attributed to IR only. Sad what the schools are producing now.

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Old 10-24-2010, 01:21 AM #5
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Default Re: Using Hoya R-72 IR transmission filter to measure IR from DPSS lasers [videos]

This is some interesting research! A bit of a reverse approach to measuring green vs IR output, with some interesting results. Personally i do IR measurements using two indentical IR blocking filters: the first to block the actual IR, and inserting the second one in the beam path to determine how much green is lost in these filters.

With these filters you'd have to measure how much IR they lose, but that could be very simple to set up. For the 808 light it would be as simple as inserting a 808 pump diode in a module and measuring its output with and without the filter in place.

For 1064 it would be a bit harder to measure, but i do not believe this is important. Virtually all laser pointer designs use intra-cavity doubling to 532. If there were any large losses of 1064 light, the whole solid state laser would not work anymore, and no green output would result either.

I think you could safely assume that the 1064 nm loss of a intra-cavity doubled 532 cannot be more than about 1 percent of pump power - if it were larger, the 1064 line wouldn't lase.
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Old 10-24-2010, 11:08 AM #6
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Default Re: Using Hoya R-72 IR transmission filter to measure IR from DPSS lasers [videos]

Benm-- Thank you for your comments! You raise the question of the consistency of the Hoya R-72 filters. Honestly I don't know how up to spec they usually are, or how the product may have changed over the years, or how much variance there is in the product line, so calibration makes sense. You suggest using a 808 nM laser for that. I think that is a good idea, but unfortunately, I don't have one, other than a pump diode from a broken < 5 mW green laser, which may not work. Even if it does, it might not have enough power for a good test, given the lack of resolution of my LPM at lower powers.

But your method with IR blocking filters can be adapted to IR transmission filters. One just uses one IR transmission filter and a stable laser with high IR emission to calibrate another, after checking that both filters block visible light. They wouldn't even have to be identical filters in this situation. (Assuming that the 1064 nM can be neglected.)

When I made my post, I had forgotten about the detailed spec sheet for the R-72 found here:

http://www.hoyaoptics.com/pdf/R72.pdf

I'll have to update my original post with the appropriate information. According to this spec sheet, the R-72 typically transmits 92% IR at both 808 and 1064 nM. Another interesting stat from this sheet: apparently the R-72 typically transmits .004% radiation at 680 nM.

Hoya doesn't give the variance in their products, but I wonder if the errors inherent in the LPMs that most members use are not a more serious concern.

I'll certainly be carrying out more experiments and reporting the results here. I have several ideas.

Thanks for the encouragement!
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Old 10-24-2010, 06:44 PM #7
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Default Re: Using Hoya R-72 IR transmission filter to measure IR from DPSS lasers [videos]

The pump diode in the broken green is likely ~ 200 mW if it's still working. That is what I use for my 808 source and I'm reading 191 mW right now.

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