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At (extremely) short range, larger, at long range, brighter.

Since the expected range of use of signal mirrors counts as "long range", the textbook answer is that the larger signal mirror creates a brighter reflection, not a larger one.

More precisely, the beam of a 3"x5" mirror is in fact 2" wider than that of the 2"x3" mirror in the largest dimension, at any range, but since the overall beam is 50 feet wide at 1 mile range, the extra 2 inches is negligible.

The reason is that each point on a mirror surface illuminated by a circular light source reflects a cone of light whose angular diameter is that of the light source. The overall beam is the summed effect of the cones from each point. If the mirror is perfectly flat, the cone from any point is parallel to the cones produced by any other point, and simply offset by the separation between the the two points. So, the maximum width of the beam in this case is the diameter of the cone plus the diameter of the mirror.

In the case of the sun the cone diameter is roughly 0.54 degrees, or a 50 foot spread to the mile. Since the dimensions of typical signal mirrors are small compared to 50 feet, the dimensions of the mirror have little effect on the shape or size of the cone at long range.

If the mirror is not flat, the beam can be wider than the sun cone, but no narrower.

This is all simplistic geometric optics, neglecting diffraction, interference and atmospheric effects.


As a rule, a quality 3"x5" plastic mirror should be brighter than a quality 2"x3" glass mirror. A 2"x3" glass mirror has less than 40% of the useful area of the 3"x5" plastic mirror, and the reflectivity of commercial plastic survival mirrors is usually (though not always) greater than 40% that of glass.

This question is a bit tougher, since the reflective quality of both glass and plastic mirrors varies with manufacture and the angle of incidence of the light, and flatness plays a large role, too. However, good mirrors are flat enough that that effect is negligible, and good plastic mirrors are more than 40% as reflective as glass.

For example, 3rd party measurements of the reflectivity of the Starflash "Ultra" mirrors ranged from 60% to over 90% that of glass mirrors. The Starflash Ultra press release of 12/13/2007 said the Ultra was "over 90% as reflective as glass", and "now 50% more reflective", which implies their standard product was "over 60% as reflective as glass".

Although a 3"x5" version of the Starflash Ultra was announced, with a quoted weight of 2.0 oz (62.2 g), AFAIK it never shipped, and after ordering several and getting the "old-style" ones, I stopped trying.

Taking the press release at face value, though, with a reflectivity > 60% that of glass for the classic 3"x5" Starflash mirror, it should outshine a 2"x3" glass mirror.

Of course, a 3"x5" laminated glass mirror should be even brighter, and they are still being made.