The basic optical structure of LED traffic lights

Since the light emitted by LED is relatively concentrated in a small solid angle range, LED traffic lights no longer need reflectors. And since LED itself emits colored light, there is no need for colored light distribution mirrors to filter the light. LED uses lenses as collimating optical components, such as convex lenses or Fresnel lenses to produce parallel light beams, and then uses pillow lenses, wedge prisms, etc. to diffuse and deflect the light beams again to produce light distribution that meets the requirements. Like traditional traffic lights, LED traffic lights also require a light shield, as shown in Figure 1.

Whether it is the European ECE, the American Ite or my country's national standard for LED traffic lights (draft for review), the light distribution of traffic lights is required to be reflected in the light intensity distribution within the HV system, as shown in Table 1.

Figure 1 Schematic diagram of the structure of LED traffic light

Table 1 National Standard for LED Traffic Lights (Draft for Review) Light Intensity Distribution Requirements

The minimum luminous flux required to meet the standard can be calculated according to formula 1:

Where: is the luminous flux in the ith solid angle region; Ii is the required (average) light intensity in the ith solid angle region; and are the boundaries of the horizontal angle and vertical angle of the fth solid angle region, respectively.

The luminous flux calculated by formula 1 is an ideal value. In fact, to meet the light distribution requirements of the standard, factors such as lens transmittance and overflow light loss must also be considered. Therefore, it is necessary to make corrections to obtain an estimated value of the actual required luminous flux. |

The light intensity distribution of LED is usually rotationally symmetric. Therefore, according to the light intensity distribution given by the manufacturer (as shown in Figure 2), the luminous flux emitted by a single LED can be calculated by the following formula:

Where: Ij is the average light intensity in the j-th annular zone, and is the boundary of the j-th annular zone.

Similarly, the value calculated here is also an ideal value, which needs to be corrected by considering factors such as temperature influence and effective utilization rate. The number of LEDs to be used can be estimated using the two corrected luminous fluxes.

Figure 2 LED light intensity distribution

In order to achieve more effective use of the luminous flux, the light emitted by the LED is first corrected into parallel light using a straightening system. The curvature radius of the convex lens commonly used is:

Where: f is the focal length of the lens; r1 and r2 are the curvature radii of the two surfaces of the lens respectively. When the surface is a plane, the curvature radius is infinite; nL is the refractive index of the lens material.

For convex lenses and Fresnel lenses of the same size and focal length, their thickness can vary greatly, as shown in Figure 3; and as the size of the lens increases, the difference in thickness also increases. The thicker the lens, the more light is lost in the process of passing through the lens, and the greater the error introduced by the thin lens approximation in the calculation.

Figure 3 Comparison of thickness between Fresnel lens and convex lens

As shown in Figure 4, the Fresnel lens is actually a "large aperture" aplanatic lens. Its optical effect is the same as that of an ordinary convex lens, but it is thinner and lighter than a convex lens. The more rings of the Fresnel lens selected during design, the more it helps to reduce spherical aberration and lens thickness, making the light spot more uniform.

In the design, lenses are used to diffuse parallel light beams to meet the requirements of the standard. Dividing the lamp cover into small rectangular units is mostly used to break up the wave surface of the light wave, which is conducive to producing a uniform appearance effect. In each small unit, a cylindrical lens is used to diffuse the light beam horizontally. After determining the unit width and the required diffusion angle, the curvature radius of the cylindrical lens is:

Where: r is the radius of curvature of the cylindrical lens; b is the width of the unit; n is the refractive index of the lens material; and the desired half-diffusion angle, as shown in Figure 5.

Figure 4 Formation of Fresnel lens

Figure 5 Schematic diagram of cylindrical lens

When determining the diffusion angle, it should be considered that the parallel light beam may have a certain period of divergence angle α. Therefore, if the total diffusion angle of the lamp is required to be 50°, it should be taken, otherwise the diffusion angle may be too large.

According to the standard, there is also a gradient of light intensity distribution requirement in the vertical direction, and it is basically below the horizontal plane. It is possible to consider using a wedge lens to deflect the light downward, and use simulation software to reasonably distribute the light flux in the vertical direction. The structure of the unit lens is shown in Figure 6. It is also possible to use a structure with arcs in both horizontal and vertical directions such as an ellipsoid or a tire surface, so that different diffusion effects can be achieved with different curvature radii in two directions. Since the standards for traffic lights generally require that the light be distributed below the horizontal plane, 9 only needs to use the upper half of the arc in the vertical direction to produce a downward diffusion effect, as shown in Figure 7.

Although two layers of lenses can better control the luminous flux, the transmittance loss of the two layers of lenses is relatively large. To obtain ideal parallel light and ideal design effects, the alignment of the focus and the determination of the position of the light-emitting point are very important. For LEDs with large luminous angles, if the collimation system uses a Fresnel lens, a total reflection structure (TIR) ​​should be used inside the edge, as shown in Figure 8. This structure can make the incident angle close to 0°, which can greatly reduce the reflection loss of the light at the edge of the lens and make the lens present uniform illumination.

Figure 6 Example of a unit lens

Figure 7 Schematic diagram of a bidirectional curved lens

Figure 8 Schematic diagram of a lens with a TIR structure

Signal lights using incandescent lamps as light sources usually use inner and outer grating sinnel lenses. A portion of the total light beam emitted by the bulb is refracted by the lens group to become approximately parallel light and projected forward, becoming an effective light beam. The spectrum of incandescent lamps is continuous, and the color spectrum emitted by colored lenses is also a continuous spectrum. LED signal lights are composed of multiple LEDs and a focusing lens array. The number of LEDs often reaches dozens or even hundreds, and the diameter of a unit lens is generally 13 to 15 mm. The spectrum line of LED is narrow and the color is pure, especially red, green, and blue light are more vivid, which is conducive to signal recognition.