Eliminating Ir Light Reflection Perceived By a Steep Viewing Angle

Eliminating IR Light Reflection Perceived by a Steep Viewing Angle

Are you having trouble with reflections on an acrylic surface in the IR spectrum? This can be a frustrating problem, especially if it interferes with algorithms that analyze objects. In this article, we will discuss a proposed solution to eliminate these reflections.

The Problem

The issue at hand is that an IR-sensitive camera with an IR filter must view an IR backlit stage below at a steep angle, with walls made of translucent acrylic. When an object approaches the surface of the acrylic, the reflection from the acrylic can interfere with the camera’s ability to detect the object. This leads to confusion as the software picks up two objects instead of one.

Proposed Solution

One proposed solution involves applying an IR filter to the outside wall to prevent the reflection from coming in and up to the camera. Alternatively, one could apply a matte to the outside of the wall to blur the reflection, leaving the inside perfectly smooth. While these options may work to some extent, there is a more effective solution that involves using an anti-reflective coating.

An Anti-Reflective Coating

An anti-reflective (AR) coating is a coating applied to optical surfaces to reduce the reflection seen when light passes through the surface. This is done by altering the refractive index of the surface, which affects the phase of the reflected light. With the right refractive index, one can effectively eliminate most of the reflection.

To apply the AR coating, the wall must be precisely ‘tuned’ for the frequency and angle of illumination. Since the angle of incidence is fixed and well defined, this is manageable. Once the coating is applied, the reflections from the surface will be much reduced, allowing for clearer, more accurate imaging from the camera.

Mathematical Considerations

The effectiveness of the AR coating depends on several factors, including the refractive indices of the surface being coated and the coating material, as well as the angle of incidence of the light. These factors can be expressed mathematically using Snell’s law and Fresnel equations, but for our purposes, we can simply say that a well-tuned AR coating can reduce reflection to almost zero.

For those interested, Snell’s Law is:

n_1 sin(θ_1) = n_2 sin(θ_2)

And the Fresnel equations are:

R_⊥ = [n_1 cos(θ_i) - n_2 cos(θ_t)]/[n_1 cos(θ_i) + n_2 cos(θ_t)]
R_‖ = [n_2 cos(θ_i) - n_1 cos(θ_t)]/[n_2 cos(θ_i) + n_1 cos(θ_t)]
T_⊥ = 2n_1 cos(θ_i)/[n_1 cos(θ_i) + n_2 cos(θ_t)]
T_‖ = 2n_1 cos(θ_i)/[n_2 cos(θ_i) + n_1 cos(θ_t)]

Where:

n_1 is the refractive index of the incident medium
n_2 is the refractive index of the transmitted medium
θ_i is the angle of incidence
θ_t is the angle of refraction
R_⊥ is the reflection coefficient for light polarized perpendicular to the plane of incidence
R_‖ is the reflection coefficient for light polarized parallel to the plane of incidence
T_⊥ is the transmission coefficient for light polarized perpendicular to the plane of incidence
T_‖ is the transmission coefficient for light polarized parallel to the plane of incidence

Conclusion

If you have the budget for special purpose equipment, we recommend using an anti-reflective coating to eliminate IR light reflection perceived by a steep viewing angle. With a precisely ‘tuned’ coating, the reflections from the surface will be greatly reduced or even eliminated, allowing for clearer and more accurate imaging.

So don’t let reflections interfere with your algorithms any longer. Invest in an anti-reflective coating on your acrylic surfaces today for better imaging results.

Eliminating Ir Light Reflection Perceived By a Steep Viewing Angle