How to choose TIR lenses for high power LEDs

1. Background

High power LEDs are widely used in our daily life now. Figure 1 shows a typical 1W LED.

Fig.1 A typical high power LED of 1W

In general, those high power LEDs have wide viewing angles, meaning that the light coming out from the LED will go towards many different directions. Figure 2 shows a typical 1W LED angular distribution of light:

Fig.2 Radiation Characteristics of a typical 1W high power LED

However, in many cases, we want the LED light be directional. For example, we want a reading light be focused on a small area just to see the book clearly, but we don’t want the light spread out all over the places. For another example, when the warning light on a police car is turned on, the light should be collimated so it can focus on a certain target, and also can be seen at long distance away. In both of the cases, the light needs to be directional. To achieve this, we have to rely on so called “secondary optics”, which means a separate optical component outside of the LED light source. With the help of “secondary optics”, we can concentrate (or focus) the widely spread light rays from an LED into a narrower angular range, so the light can go more directionally.

There are two ways of focusing light: using reflector or using refractive lens. A typical example of reflector is the one you can often find in an old flashlight. In this case, the large angle light rays will be reflected by the shiny surface of the reflector, and be re-directed to a certain direction. On the other hand, a refractive lens uses the refractive index change at the interfaces of the lens and the surrounding media (typically air) to change the direction of the light rays, following Snell’s law. For example, we all know that if we put a point source at the focal point of a lens, we can get a collimated light beam at the other side of the lens.

2. What is TIR lens?

TIR lens (sometimes also called TIR reflector) is a combination of the reflective and refractive approach. For the reflective part, instead of using a front surface reflection like a traditional reflector, it uses Total Internal Reflection (TIR). As shown below, the center part of the lens uses refractive optics to focus the light, and the rest part of the lens uses TIR from the interface of the lens and the air to re-direct the light.

Fig.3 Principle of TIR lens

TIR is an optical phenomenon that occurs when a ray of light strikes a medium boundary at an angle larger than the critical angle with respect to the normal of the surface. If the refractive index is lower on the other side of the boundary, no light can pass through, so effectively all of the light is reflected.

3. Different types of TIR lens

TIR lenses are characterized by how wide a beam they produce. The angular width the TIR lens produces is usually specified by measuring the angular separation between the directions, at which the intensity has fallen to half its peak value. The value is called the Full Width Half Maximum (FWHM) divergence.

Fig.4 Full Width Half Maximum Angle definition

Since LEDs from different manufacturers are of different shape and optical materials, the spatial and angular distribution of the light rays are different. In order to achieve the best performances, the TIR lenses have to be specifically designed for each type of LEDs.

When mounting secondary optics, positioning the optics at the correct height relative to the LED is essential if you are to obtain best efficiency and the correct beam width. Equally important is the alignment of the optic axis to the LED chip. If not correctly positioned, the output beam will become uneven and offset.

However, manufacturers also provide general purpose TIR lenses. Although they are designed for some certain type/brand of LEDs, they can be used with many other brands of LEDs and the performances are good enough for general applications.

Shown below are the light beam pattern of a 3w green LED with different TIR lens. The images are captured at about 2 meters away from the LED.

8 degree TIR lens: