Milliwatts explained: what the power figure on a laser pointer means

Two green pointers sit next to each other on a comparison page. One says 5 mW, the other says 50 mW, and the second costs a good deal more. The power figure is the only thing that visibly changes, so it looks like a score out of ten.

It is not a score. The mW figure is optical output power in thousandths of a watt: how much light leaves the front of the pointer each second. It says nothing about colour, and colour decides how much of that light your eye registers. A milliwatt of green is worth roughly eight milliwatts of red.

Read the power figure and the wavelength together and picking a pointer takes a minute. Below: what a milliwatt is, what climbing the range buys, and where more power stops helping.

What a milliwatt actually measures

The mW rating is optical output power: the energy the beam carries past the front lens each second. “Milli” is the SI prefix for one thousandth, so 1 mW is 0.001 W and 1,000 mW make a full watt, as the NIST Office of Weights and Measures sets out. A pointer marked 50 mW emits 0.05 W.

Two things follow. The first is that the number is an output, not an input. A green 532 nm pointer reaches its colour in three steps, according to Wikipedia’s laser pointer entry: an 808 nm infrared pump diode of typically 100 to 300 mW drives a neodymium doped crystal lasing at 1064 nm, which is then frequency doubled to 532 nm. The figure on the front is what survives those conversion steps, not what the pump diode started with.

The second is that a watt measures energy, not appearance. A watt of infrared and a watt of green light carry identical energy and look nothing alike, because one of them your eye cannot see.

mW, nm and lumens: three numbers, three jobs

Figure	Unit	What it tells you
50 mW	Milliwatt, thousandth of a watt	Optical output power: light energy leaving the pointer each second.
532 nm	Nanometre, billionth of a metre	Wavelength, which is the colour. 532 nm is green, 650 nm is red.
30.2 lm	Lumen	Visible output: power weighted by how well the eye responds to that colour.

Specification sheets quote the first two and almost never the third, which is why two pointers with the same mW figure behave differently.

Why colour decides what a milliwatt is worth

Your eye is not an even detector. It responds strongly to yellow green and weakly to deep red and violet, so identical power at two wavelengths produces very different amounts of visible light.

The conversion is fixed by definition. The BIPM defines the candela by taking the luminous efficacy of radiation at 540 x 10¹² Hz to be 683 lumens per watt, the eye at its most responsive. Every other wavelength scales down from there by the CIE photopic curve. The 1 nm table from the Colour and Vision Research Laboratory at the UCL Institute of Ophthalmology gives 0.8850 at 532 nm, 0.2170 at 635 nm, 0.1070 at 650 nm, 0.0610 at 660 nm and 0.0298 at 445 nm, with the peak of 1.0000 at 555 nm.

Multiply 683 by those values and the gap is plain: 604 lm/W for green at 532 nm against 73 lm/W for red at 650 nm, a factor of 8.3 at the same rated power. Which colour suits which job is covered separately in which laser pointer colour to choose.

The dot and the beam are two separate questions

Everything above concerns the dot on the wall. Seeing the beam itself in the air is a separate calculation, and it is the one that catches buyers out on an outdoor pointer.

A beam shows from the side only because a small share of it scatters off air molecules on the way. Rayleigh scattering is inversely proportional to the fourth power of the wavelength, per Wikipedia’s Rayleigh scattering entry, so shorter green wavelengths scatter far more strongly than longer red ones. The same entry puts the nitrogen cross section at 5.1 x 10^-31 m² at 532 nm, which with roughly 2 x 10²⁵ molecules per cubic metre leaves about one part in 100,000 of the light scattering out of the beam per metre travelled.

That tiny fraction is why a beam line needs both the right colour and a decent power figure. Night vision shifts the balance further: the dark adapted curve in the same CVRL data set peaks at 507 nm and reads 0.7810 at 532 nm against 0.000677 at 650 nm. It is the reason green is the standard choice for stargazing and pointing out constellations.

From 5 mW to 50 mW: what changes and what does not

Ten times the power gives ten times the visible output, in a straight line. Doubling the milliwatts doubles the lumens and nothing else: colour, spot size at distance and beam shape all stay put.

The green column below is 683 x 0.8850 x the power in watts, the red column 683 x 0.1070 x the power in watts, using the BIPM and CVRL figures.

Rated power	Green 532 nm	Red 650 nm	What moves
1 mW	0.60 lm	0.07 lm	A readable dot indoors and on a screen.
5 mW	3.02 lm	0.37 lm	Green holds up outdoors at dusk; red needs a dark surface.
10 mW	6.04 lm	0.73 lm	Green dot stays findable across a yard in daylight.
20 mW	12.09 lm	1.46 lm	Green beam line starts to show in clear night air.
50 mW	30.22 lm	3.65 lm	Green beam readable at distance in low light.

One comparison there is worth memorising: 5 mW of green gives 3.02 lumens and 50 mW of red gives 3.65, so a mid range green pointer and a ten times stronger red one land in the same bracket.

Matching the power figure to the job

Start from the distance you work at and the light you work in, then read the figure off. Indoors almost nothing is needed; outdoors the dot has to compete with a bright background.

One job sits outside this ladder. Moving birds off a site means reading a beam that carries several hundred metres in low light, which is a different class of tool from a pocket pointer. What that work asks of a beam is set out in the guide to using a green laser pointer as a bird scarer.

What more milliwatts will not give you

Power does not shrink the dot. Spot width at 50 m or 500 m comes from beam divergence, set by the optics rather than by the output figure.

Divergence is the angular increase in beam diameter with distance, usually quoted in milliradians. The teaching notes from Amrita Virtual Labs give it as theta = (w2 minus w1) divided by (z2 minus z1), the change in beam width over the change in distance, with a minimum set by diffraction at theta = lambda / (pi x w0), where w0 is the waist radius at the output. Neither expression contains the power figure.

So a stronger pointer at 100 m gives a brighter spot of the same width, not a tighter one. A soft dot at distance is not something the figure on the box will fix. Because divergence falls as the beam leaving the lens gets wider, a well collimated 10 mW pointer can hold a neater spot at range than a poorly collimated 50 mW one.

Frequently asked questions

What does mW mean on a laser pointer?

It is the optical output power in milliwatts, one milliwatt being a thousandth of a watt under the SI prefixes published by NIST. A 5 mW pointer emits 0.005 W at the lens, a 50 mW pointer 0.05 W. It measures the light leaving the device, not the power the batteries supply or the brightness you perceive.

What is the difference between mW and nm on a laser?

They are unrelated. mW is power, how much light comes out. nm is wavelength, which is the colour: 532 nm reads as green, 650 nm as red. You need both, because the same power at two wavelengths gives very different visible output. Only the pair together predicts what the dot will look like.

Is 50 mW much stronger than 5 mW?

In output terms, exactly ten times. In the same colour that means ten times the lumens: 3.02 against 30.22 for green at 532 nm on the CIE figures. It does not change the spot size at distance or the colour. It is a brightness step: worth it outdoors, pointless in a meeting room.

Why does a green laser look brighter than a red one at the same power?

Because your eye is far more responsive to green. The CIE photopic curve published by the Colour and Vision Research Laboratory reads 0.8850 at 532 nm against 0.1070 at 650 nm, so at equal power green delivers 8.3 times the visible output. After dark the gap grows: 0.7810 against 0.000677 on the dark adapted curve.

How many mW do I need for a laser pointer?

Match it to distance and daylight. Indoor pointing at a screen needs very little. Outdoors in daylight across a yard, green in the tens of milliwatts keeps the dot findable. For a beam line in the night sky you want green again, higher up the range. Open ground at several hundred metres asks more of a beam than a pocket pointer will give.

Reading a spec sheet from here

Take the two numbers together and in that order. Fix the colour first, because green at 532 nm buys 8.3 times the visible output of red at the same power, and extra milliwatts on a red pointer do not close that gap cheaply. Then set the power from the distance and the daylight you work in, and expect a brighter dot rather than a smaller one as the figure climbs.

With those two settled, the rest of a listing is detail. Every model in the green, red and violet pointer range is listed with its colour and its wavelength, so you can settle the first of the two numbers before you look at anything else.

Tim Buikema runs Emax Laser Pointers, the UK shop of Emax Lasers. He has supplied laser pointers to farms, marinas and facility managers since 2011 and opened the UK shop in 2017.