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Home Made Laser Power Meter
The outputs of laser pointers are limited according to the Class they are defined under. You might expect then that the outputs of all lasers of a particular class would be similar. Not so!. If you’ve played with a few, you will have noted that some have much better output than others.
If you use your laser for Holography and certain other applications, you might want to choose a device with high output. It is difficult to make such a judgment by eye. Further, since lasers’ beam focus differ, differences in total light output become even more difficult to judge.
For advanced enthusiasts who want to include electronics to increase the output of an individual laser (see a later article), it becomes imperative to measure the brightness so that catastrophic and near-instant destruction of the laser diode does not occur.
For these reasons, it is helpful to be able to make an objective measurement of laser output by a method that is repeatable and insensitive to beam focus. Absolute measurement, in terms of mW per square cm. for example, is not necessary but accurate relative measurement is.
These notes describe a photometer that can provide such relative measurements. In fact the device, which can be easily assembled from readily obtainable parts, is used in conjunction with an inexpensive digital MultiMeter CLICK Photos for Close ups |
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The photometer consists of just a target that the laser can illuminate, a photo detector and a container to place these in and exclude other light.The target is made of matt white paper, the photo detector can be a commercial photo-transistor or a cheap home-brewed alternative (see later note) and the container might be a plastic 35 mm film package or a pill box. |
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This pill box has a 3.5 mm jack fitted in the lid. This allows more convenient connection to the MultiMeter. The jack inside also provides a convenient mount for the photo cell. The laser beam is shined through a hole in the lid. In order to contribute to a consistent aim, part of the exterior of a DIN plug has been glued on to act as a guide. |
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Note that the photo transistor has been soldered directly to the jack plug and that only two of it''s leads have been used - the base and the emitter. When light falls on the photo-transistor, electrons are displaced and that results in a voltage being generated across the base-emitter junction.
Note that a resistor has been fitted in parallel with the photo-transistor. This is not in general necessary. It was fitted in the prototype to reduce it''s output to a value more convenient for the MultiMeter being used.
Try to find a container made of opaque material. This will avoid the need for a kitchen foil lining as black out material. Why not pop out for some vitamin B tablets? Choose them for the colour of the box. The target is made of a curved piece of heavy matt paper placed in the bottom of the container. If the container is not opaque, then a lining of kitchen foil can be used to exclude extraneous light. Any gaps should be painted over with matt black paint.
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| There is just one tricky stage in the construction. That is bending the wires of the photo-transistor so that it looks in the best direction. This might well take some experimentation.
For greatest sensitivity, it''s obvious that the photo-transistor should point at the place where the laser beam will fall. This is not as easy to arrange as you would expect because the device may not be most sensitive along it''s axis. Also you can''t bend the wires while it''s inside.
Another difficulty is that it is better if the area of best sensitivity does NOT match the laser beam target too exactly. This is because lasers differ in focus. You need the photo-transistor to be equally sensitive to light falling an area larger than expected. Having a larger sensitive area will make the photometer insensitive to variations in laser focus and the inevitable aiming errors. Therefore, you will need to experiment with changing the direction that the photo-transistor is pointing and its distance from the target.
If you can get readings that vary very little while you waggle the laser around in it''s DIN-plug guide then the adjustment is good. The readings will not be the highest when it is optimally adjusted.
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Having completed the adjustments you can measure all the lasers that come to hand. Lasers that measure low but look bright might well be producing light nearer the orange part of the spectrum. Not easy to judge by eye but easily discriminated with a spectroscope. (see the notes about making your own).
Some later notes will discus methods of measuring laser output as a function of drive current. This can determine the maximum safe (for the laser) output of the laser diode. Electronic circuitry to provide the custom drive for each laser will be described. You''ll need VERY good eyesight if you want to build the electronics into a laser pointer though.
Note on Home-brewed photo-transistors.
Silicon photo-transistors are not so special. In fact all transistors are inherently light sensitive and have to be packaged so as to exclude any light. Photo-transistors however are manufactured with windows to let the light in and have well specified photo characteristics. You can make your own photo-transistor by hacking out a window in an ordinary silicon transistor.
This is most easily done with a metal can device such as the BC109C. The posh name for this kind of packaging is TO-18. BC109s are manufactured by many companies and are available from retailers such as Maplins and CPC. The BC109 transistor belongs to a family including BC107, BC107B, BC107C, BC108, BC108B, BC108C, BC109 and BC109B. Any of these will be suitable.
A BC109C looks like this before the window has been fitted:
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To open the device simply file the lid off. The metal is quite thin and you''ll get through it quite quickly. The transistor will be a bit awkward to hold. When the lid has gone the device looks like this:
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If you want, you can protect the device with epoxy resin. Mix up a dollop and fill the cavity. Pile it high-ish - ice cream cone style and hold the device upside down while it sets (choose a quick curing epoxy) and it forms a rudimentary lens like this: |
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If time permits, mix the epoxy by very gentle folding. This will prevent the ugly bubbles you see above from being formed. It will look nicer but good optical performance is not required.Finally, you need to identify the base and emitter connections. The leads form a long based isosceles triangle. With the Base at the Bottom, the Collector comes first in clockwise rotation - BBC :Remember, you only need the base and emitter connections. Enjoy!
Hope you find this useful and fun. |
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Calibration of your Homemade Laser Pointer Power Meter |
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Your Home made Meter produces results that do not relate in any way or form to the Milliwatt output the lasers produce but if you can obtain at least two verified laser outputs you can work out a scale of Power in relation to your meter. Your meter Incidently is very unlikely to produce results that are Identical to these and may even vary wildly from these but do not be concerned as long as there is a Variable you can predict the output pretty accurately. |
| We used many different lasers to obtain a Calibration on above meter , because we have many ! So we feel its pretty accurate but all you really need is two lasers of different output that you feel you can rely on to predict the Result from the meter in to a Milliwatt power. The hardest thing for most people is to find two different output lasers were you feel you can reliably believe the badged outputs . As a general rule the more expensive the laser the more likely that it will match the badged output Power.
Heres Some results we recorded
Tested random three random red pointers ... ( Badged at <1mW)
1.- 4.7
2.- 6.1
3. - 5.7
As a general rule I dont normally open THE Greenies , to provide
in pristine condition without finger marks but here goes. Each was
tested for 1-3 minutes to provide testing warming up period
Opened SIX Green Pointers
3 x Normal <1mw
1.- 5.2
2. - 4.6
3. - 5.5
3x Enthusiast <1mW but containing 5mW diode.
1. - 4.3
2. - 3.6
I think Ive checked enough here to take the conclusion that a reading of
between 4 to 5 is a UK legal output of <1mW on my meter. Agreed ?
Now tested Units badged with Higher Power to see if results bore any relation to the Increase In Power
5mW. - 22 / 15mW. - 70
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These readings proved very accurate for a Home made device and allowed the Measurement of the Tweaked £ 7.50 ( Class II <1mW ) laser showing in Photograph to 81.6 which relates to an output Power of 17mW , this is excellent going for such a budget laser but please be aware there was a lot of work Involved in Tweaking this laser , indeed years of expertise and Knowledge | |
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