Heat, Moisture and Mirrors: How to Avoid the Funhouse Effect


The plastic used in mirrors (typically acrylic or polycarbonate) is a thermal insulator. This means that when there is a change in temperature, one side of a piece of plastic may be warmer than the other, and it takes some time for the heat to soak through and equalize the temperature throughout. In the meantime, when there is a temperature gradient in the plastic, the plastic bends to accommodate the temperature inconsistency within.

How Does Temperature Impact Flat Mirrors?

When you start out with something flat, even the slightest amount of curvature produces visible and noticeable image changes due to the variation in the radius of the curvature, either convex or concave. Replex Plastics maintains that it’s better to know one’s limitations than to raise false expectations of performance, and the fact is that it’s very difficult to keep flat plastic mirrors perfectly flat!

How Does Temperature Impact Convex Mirrors?

Convex mirrors endure length changes as well, but the pre-existing curve makes such changes generally imperceptible to the human eye without the aid of a measuring apparatus.

How Does Moisture Impact Acrylic Mirrors?

Moisture also contributes to changes in length — and therefore curvature — in acrylic mirrors. Both absorption and desorption of water in the acrylic can result in curvature changes similar in magnitude to that which would result from temperature shifts.

In mirrors with aluminum on one side, all moisture is absorbed from the front (not the metallized) side, and it can take weeks or even months to reach equilibrium within. In the meantime, the moisture inconsistency results in changes in curvature. As with temperature-induced changes, these changes induced by moisture are most noticeable in flat mirrors , and much less so with convex mirrors.

Choosing the Right Housing

It is also critical to choose the right housing for the mirror. Since plastics can grow and shrink at four to six times the rate of most metals, an acrylic mirror is better served by a plastic housing, as opposed to a steel housing — unless it has sufficient edge clearance to allow for thermal expansion and contraction of the acrylic.

We find that Replex’s clients make the best design decisions when they know the strengths and limitations of the materials with which they’re working. Replex is here to help our clients succeed and can provide more detail as needed for your application.

Contact Us

11 Mount Vernon Ave
Mount Vernon, OH 43050 
1-800-886-8847 (U.S.A Only)

 Want More Information on Replex Products?

Whether it's a transportation mirror, safety and security mirror, plastic dome, or other custom products, we're happy to answer all your questions. 

Replex Reads

Sign up to receive monthly updates from Replex!