Variable spot size + color mixing optical system

Published by Anatoli Trafimuk.

In applications such as concert show lighting, architectural lighting, and cinema projection good color mixing is essential. This means the design of optics in front of a RGBW LED source must include optimization for color uniformity as well as beam shape.

A common solution is to place diffuse materials in the optical path. This can provide good color mix but has negative trade-offs including increased etendue and increased light losses. These schemes are not suitable when tight beam angles are required.

One design approach that can be used for narrower beam angles is color mixing rods. In the images below that approach will be explained.

A typical RGBW LED shown in the first image, knowing die location in LED is important. Some LED suppliers use mixed die location in their products for example Ledengin LZC-03MD07 LED. In that arrangement the diffusion option is the correct approach.

RGBW LED

RGBW LED

A simple glass or clear plastic rod produces even color illuminance at the end surface if we place LED at opposite aperture. But the luminous intensity is poorly color mixed. The light is collimated but the luminous intensity profile is also transferred. See images below

Clear plastic rod produces even color illuminance at the end surface

Clear plastic rod produces even color illuminance at the end surface

Luminence and illuminance after the mixing rod

Luminence and illuminance after the mixing rod

By adding a microlens array at the each end of the rod, we can achieve mixed color luminous intensity.

A microlens array at the each end of the rod gives mixed color luminous intensity

A microlens array at the each end of the rod gives mixed color luminous intensity

High angles of light at the end of mixing rod

High angles of light at the end of mixing rod

But we still have high angles of light at the end of mixing rod. Some amount of light is reflected back due total internal reflection on steep microlens surfaces. Light is lost. To increase efficiency we add some slope to the rod. Color mixed spots is improved. See images below.

Some light is reflected back due total internal reflection on steep microlens surfaces. Light is lost

Some light is reflected back due total internal reflection on steep microlens surfaces. Light is lost

To increase efficiency we add some slope to the rod. Color mixed spots is improved

To increase efficiency we add some slope to the rod. Color mixed spots is improved

With slope, we improve color mix and efficiency

With slope, we improve color mix and efficiency

Color mix spot with good luminous intensity

Color mix spot with good luminous intensity

Now that we have good luminous intensity color mix, we can use a TIR lens to collimate light. And can use asphere based concept to provide variable spot system. The spot shown here is not perfect, but can be improved with additional optimization, or adding more lens elements.

Using a TIR lens to collimate light

Using a TIR lens to collimate light

Using asphere based concept to provide variable spot system

Using asphere based concept to provide variable spot system

This approach can have higher cost of manufacturing compared to others and depending on volume because it required tighter tolerances of optics and mechanics, but overall it provides a good trade-off between efficiency, quality and price.