Spherical vs. Parabolic Mirror Focal Regions

Spherical vs. Parabolic Mirror Focal Regions

Please Wait


Why a Parabolic Mirror Instead of a Spherical Mirror?


Spherical Mirror Ray Trace with Focus Indicated
Click to Enlarge

Figure 2: Spherical mirrors do not reflect all rays in a collimated beam through a single point. A selection of intersections in the focal volume are indicated by black dots.

Parabolic Mirror Ray Trace with Focus Indicated
Click to Enlarge

Figure 1: Parabolic mirrors have a single focal point for all rays in a collimated beam.

Parabolic mirrors perform better than spherical mirrors when collimating light emitted by a point source or focusing a collimated beam.

Focusing Collimated Light
Parabolic mirrors (Figure 1) focus all rays in an incoming, collimated light beam to a diffraction-limited spot. In contrast, concave spherical mirrors (Figure 2) concentrate incoming collimated light into a volume larger than a diffraction-limited spot. The size of the spherical mirror's focal volume can be reduced by decreasing the diameter of the incoming collimated beam.

Collimating Light from a Point Source
A point source emits light in all directions. When this highly divergent light source is placed at the focal point of a parabolic mirror, the output beam is highly collimated. If the point source were ideal, all reflected rays would be perfectly parallel with one another.

When a point source is placed within a spherical mirror's focal volume, the output beam is not as well collimated as the beam provided by a parabolic mirror. Different rays from the point source are not perfectly parallel after reflection from the spherical mirror, but two reflected rays will be more nearly parallel when they reflect from more closely spaced points on the spherical mirror's surface. Consequently, the quality of the collimated beam can be improved by reducing the area of the reflective surface. This is equivalent to limiting the angular range over which the source in the focal volume emits light. 

Choosing Between Parabolic and Spherical Mirrors
A parabolic mirror is not always the better choice. Beam diameter, cost constraints, space limitations, and performance requirements of an application all influence selection. Beam diameter is a factor, since the performance of these two mirrors is more similar when the beam diameter is smaller. Parabolic mirrors are more expensive, since their reflective profiles are more difficult to fabricate. Parabolic mirrors are also typically larger. Improved performance may or may not be more important than the difference in cost and physical size.

Looking for more Insights? 
Browse the index.

Date of Last Edit: Dec. 4, 2019

Posted Comments:
Andy Jiao  (posted 2023-11-30 18:57:07.497)
For the parabolic mirrors, there is an article explaining the difference between parabolic mirrors and spherical mirrors. Can there be an article also explaining the differences of using parabolic mirrors and spherical lenses for focusing?
cdolbashian  (posted 2023-12-15 09:33:05.0)
Thank you for reaching out to us with this request! We have added this to the list of future lab projects/articles. Perhaps in the near future, you will see this presented! In the meantime, I have contacted you directly to inquire about your specific curiosities.