# Laser Ratings

## Introduction

Presently it is easy to buy very bright laser pointers, and the market is full of false claims of power output since this is an easy way to jack up the price. At higher laser powers especially, the eye does not distinguish well between various beam powers. On the one hand, this suggests that power is probably not the best measurement for laser pointer selection, and instead apparent brightness is better (which takes into account the eye's sensitivity to certain wavelengths and certain beam diameters). Nonetheless power is a measure that is objective and can be determined with a meter. Electrical power is easy to measure, but optical power requires a special sensor. I had a chance to experiment with a laboratory optical power meter, and some findings of the actual power output of typical laser pointers are shown below.

## Safety

While on the topic of laser power, I would like to throw in a word about laser safety. A laser beam presents a safety challenge unlike bright flashlights / staring at the sun since the energy of the laser is in a fairly tight beam (it is contained in a small area even at large distances from the laser source). When energy per cross section area is considered, even a weak laser beam directed into the eye is similar to staring at the sun (sun = 1360 W/m2, 5mW laser beam with area 2x2mm = 1250 W/m2). This property is unlike the majority of light sources we experience every day - generally if a light is too bright it is possible to move or look away from it and reduce the intensity of light hitting the eye. For example, when using a very bright flashlight at night, it is best to point it at far away objects so as to not decrease the eye's sensitivity to light. This logic does not work with lasers, since even a far away object can reflect the laser beam directly into the eye, delivering all the energy to a specific spot. Similarly, one might try to get an idea of how bright a flashlight is by looking at it from a certain angle off the main beam axis, so that only some of the light output is visible. This approach will be very inaccurate for a laser, since again nearly all the light output will be solely in the forward directed beam. As mentioned above, the eye is not well suited for evaluating laser beam power, which is the actual measure of how dangerous a laser beam is (particularly if the laser is invisible). Thus for work with higher power lasers (100mW and above), goggles which block the laser's wavelength are a must. With medium power lasers (10-100mW), such as those generally available on the market, it is very important to ensure no reflecting objects will be present in the laser's path (including mirrors, metal (even unpolished!), glass (such as in windows), water, transparent plastic) so that the reflected beam will not enter the eye. It is imperative to not shine the laser near any other people or animals, since at this power the laser is not a toy. Lower power lasers (10mW and under), such as typical red pointers, are still very unpleasant if directed into the eye but generally at low power the blink reflex is fast enough to prevent damage to the eye.

## Measurements

All measurements were carried out using a Thorlabs S121C sensor (measures up to 500mW) with the wavelength set to the one on the laser pointer label (this introduces inaccuracies if the laser is putting out multiple wavelengths, which I did not have the equipment to test). The laser pointers were used with a freshly charged battery and were measured about 30 seconds after being powered on. (Update Nov. 21, 2015: I also tested reflecting them off a mirror, which was probably aluminum. This was a front-coated mirror so a common back-coated mirror will have a lower reflected power. I also repeated the initial test procedure and got the same results.)

In the series of measurements, I compared the lab's 445nm diode as a reference, a typical "5mW" red laser pointer, three readily available high powered laser pointers advertised at "<1000mW" - red green and violet, and just for fun a pretty bright LED flashlight.

 DUT Wavelength Claimed Optical Power Actual Optical Power Reflection off mirror Comments Lab Diode 445nm <500mW Tested to 400mW Not tested At 100mW, can feel heat on skin with collimated beam Standard Red Pointer 650nm <5mW 0.5mW 0.47mW (94%) Just enough for projection use High Power Red Pointer 650nm <200mW 82mW 65mW (79%) Beam visible in very dark room High Power Green Pointer 532nm <1000mW 65mW 30mW (46%) Beam visible in dark room High Power Violet Pointer 445nm <200mW 95mW 28mW (29%) Beam visible in very dark room LED Flashlight Broad ? 100mW Not tested Advertised at 170 lumens

## Discussion

We see that the violet pointer (the smallest by size of the three high power ones) actually had the highest power output, followed by the high power red and green. When the actual laser dots are seen side by side, this is hard to believe. Green appears to be the brightest by a wide margin, followed by violet and then red. This points to a flaw in using optical power as a measure of perceived brightness - the eye has different sensitivity to different laser wavelengths, and green is one of the most sensitive regions. Thus the green laser pointer appears very bright at 65mW, and this is enough for the manufacturer to put on a "1000mW" sticker as opposed to "200mW" for the red and violet pointers. In a way, it is safer to buy the green laser pointer if your goal is to have very bright projections, since then your beam will have less power while still appearing very bright. This is also why invisible lasers can be dangerous.