Light source is a primary component of this system, and the correct choice will facilitate the process of setting up the system. This part is made of a green diode laser with a wavelength of 532 nm, and its output power is 1 watt, which is being control by using a mechanical shutter. The output light intensity is controlled to create different levels of brightness. Dimensions of used laser are 40(W) × 36(H) × 90(L) and its specifications are according to Table 1. Inherent parallel beams of laser light placed in the light cone are faced to the image source thus creating minimal waste and desired thumbnail display clarity.

This light source is reliable, highly durable, with a very limited and narrow wavelength range, and has desired brightness for avionic applications. Its radiation safety measurement at field of view is critical; this will be

calculated in terms of these parameters: the output light intensity, wavelength range and photo of perception [15] . The bandwidth of this laser and losses caused by intermediate elements (optical or electro optical elements) will cause the final light intensity to become safe. In addition, the light intensity can be adjusted for various applications such as day and night and high or low light outdoor intensity. Laserability to couple with DMD micro display in higher efficiency than other light sources made it an ideal choice to use in our system.

As mentioned in section one, the selected image source type is DMD, the DMD is fabricated in c-si using similar processing technologies as those applied to integrated circuit fabrication. It consists of an array of mirrors, one per pixel, suspended over CMOS circuitry. Figure 2 shows photographs of the device. In Figure 2(a), the micro mirrors are shown. In Figure 2(b), one of the micro mirrors has been removed to show the underlying structure. Figure 2(c) shows a magnified view of this underlying structure.

The mirror assembly consists of a square mirror on top of a post attached to a torsion-hinge-suspended yoke. The yoke can rotate about ±12˚ (±10˚ in older DMDs) before it contacts the mechanical stop. For this deflection, the torsion effects in the hinges are fully elastic. On the silicon substrate, push-pull address electrodes are connected to the CMOS circuitry. The CMOS electronics in the underlying silicon substrate consists of a six transistor SRAM cell per pixel plus other circuits. With the appropriate bias, the mirrors move in response to the voltages on the address electrodes. In the +12˚ position, light incident onto the mirror will be reflected on to the projection lens, and in the −12˚ position, the light is directed away from the lens input and is internally absorbed. Thus, the light from any position on the mirror array is either present or not present, i.e. the projected intensity is binary. The mirrors can switch positions in about 15 μs, which allows achieving gray scales by time-multiplex- ing the mirror position. Luminance between the full-on and full-off values is obtained by selecting the appropriate on- and off-times. Furthermore, DMD micro display has other benefits: first a reflection structure of micro display caused the surface to become warm later, which can also cause high thermal power.

The final part of HUD system design requirement is the optical design. Optical design is divided in to two parts: first; adapting light module, and second: projector module. The first module is used to make a proper illuminating light profile and the second part is used to make a perfect magnified final image.

Although the use of laser light is very useful, its output light profile is Gaussian. Since making a uniform image in the whole field of view is a very important consideration, this problem should be eliminated as much as possible.It is possible to design an adapting light module; which is composed of two elements: diffractive optic and beam expander element. Diffractive optic element shown in Figure 3, changes the Gaussian profile of laser light to top hat ones that have a high uniform distribution [17] . This uniform light is necessary to make a fit image in the final surface. Its specifications are according to Table 2.

The second necessary element is beam expander; which consists of two 1:5 and 5:7.8 mm element shown with 2# and 4# markers in Figure 1. The first, 1:5 mm element makes entrance light beam for diffractive element and the second 5:7.8 mm makes entrance light beam for beam splitter type prism. These designs are diffractive limited, and no more aberrations are added to its output according to Figure 4 and Figure 5, respectively.

This module is the main optical element in this electro-optical system. As shown in Figure 6. Number 1 element

represents the effective area of the used DMD micro display. This element is considered a circle form with a diameter of 7.62 mm to cover up the maximum area of this micro display. Number 2 element represents the beam splitter prism. This prism is made of SF2 material and is considered as a cubic glass with 25.4 mm dimensions. This prism is considered as a transparent cube in optical design. Numbers 3 - 5 and 7 - 11 elements represent spherical lenses.

Number 6 element represents the system’s stop. Nominal stop is the metal diaphragm with 26.0 mm inside

diameter that can be changed up to 3.0 mm tolerances to illumination settings. Number 12 optical element is added to correct aberrations for convergence of projection module’s output radiation. The tube length is equivalent to 173 mm that is equal to the distance from the DMD micro display to diffuser display screen, which represents a compact projector system. High compact optical element creates problems in its design. The most important problems are; excessive aberrations, extreme changes in image brightness level and excessive f/#, reduction aberrations techniques, and optimization of these three factors will be presented. The overall features of the designed system are: the length of the projector is equal to 173 mm that is highly desirable and represents a compact system, this module’s optical zoom is 10X, which leads to a 76.2 mm diameter image, it includes 9 spherical lenses. The great benefit of this design compared to other types is its simplicity in manufacturing, assembly, test process and its being more economical. Using a DMD micro display reduces light losses in secondary reflections from its surface, and the image spot diameter created by the projector must be less than 120 μm according to CRT spot diameter. This amount is equal to 15 μm and is approximately 8 times as good as defined value. Important results of our designed module listed in Table 3.

Analysis related to image quality are as follows: the maximum spot diameter of the created image is equal to 15.4 μm according to Figure 7, the distortion aberration amount in the full range of image display is less than five percent and field curvature aberration in the full range of image display is less than 5 mm. According to Figure 8, point spread function for this design is shown in Figure 9. As shown in this figure, an optimized design is carried out.

Figure 10 shows diagrams of relative brightness depending on the height of the beam. As seen the maximum variation in the whole field of view is 5%, which is very desirable. Here is a reminder point: as mentioned later, the gain type diffusers were used to create more uniform illumination in the field of view. Performance of this diffuser image surface is so that the light intensity in the corner is less than other points, and this phenomenon is corrected by an optical design trick. The polymeric elements present interesting abilities as a fit and designed diffuser display surface in a particular field of view is grossly different from the other parts; and therefore, without the excessive brightness from the lighting source, you can easily build a clearer picture in your desired viewing angle.