Principle of mobile phone projection
Whether it is a real projector or the mobile phone projection box we are going to make today, the principle is actually very simple. That is "convex lens imaging", which was introduced in middle school physics: when an object is placed outside the focus of a convex lens, an inverted real image is formed on the other side of the convex lens. There is also a formula: object (distance), far image (distance) Near is small, objects near are large as far away. A projector changes the size of the projected image by adjusting the distance between the lens and the projection source.
Principles of projectors
Speaking of projection sources, there are currently many types of projectors, the most common ones being LCD and DLP. DLP (Digital Light Processing) technology is patented by Texas Instruments. Its core is a digital micromirror chip, which is a chip arranged with countless "small mirrors". Each pixel corresponds to a "small mirror" , it does not rely on changing the polarization state of the incident light or its own luminous state, but on adjusting the reflection angle of the micro-mirror at a specific position - if you need certain pixels to emit light, drive the small mirror to reflect the light toward the lens, thereby An image appears on the screen. Obviously, the way DLP works is too high-end and not suitable for DIY production. The LCD projector technology is much simpler, especially the principle of the single-chip LCD projector that many DIYers often use now: use the LCD screen as a light-transmitting screen to directly project the image onto the screen through the lens.
So, I use a mobile phone as the projection source, so that I don’t have to consider the light source, heat dissipation and other issues. The only regret is that the brightness of the mobile phone screen is not too high. After scattering and lens refraction, the projection is The brightness is limited and can only be used in darker environments.
Design and material selection
Based on the above principles, we can extremely simplify the "projector": use the mobile phone as the projection source, the lens uses a single convex lens or a Fresnel lens, a reflector to correct the inverted image, and A small and lightweight box will do the trick.
Projection source - mobile phone
The resolution of the LED projector is determined by the resolution of the LED screen. Use a mobile phone as the projection source of the LED projector. A mobile phone with a large screen, high brightness and high resolution is necessary. I used the OPPO Find 5 mobile phone sponsored by OPPO as the LED projection source. It has a 5-inch 1080p screen with a resolution of 1920×1080 and a pixel density of It reaches 41ppi (Pixels per inch, the number of pixels per inch). After refraction and magnification through the lens, the advantage of fine image quality can be better reflected. Since we want to make a projector that uses projected light sources, all light sources come from mobile phones. The light source of the screen itself, and the screen brightness of the OPPO Find 5 mobile phone reaches 480 lumens, and the projection effect is still good. In addition, the entire front of the OPPO Find 5 mobile phone is almost occupied by the screen, and the frame is only 3.25 mm, so the box can be made as small as possible , reducing light scattering, very suitable as a mobile phone projection source.
Lens
Because the structure is greatly simplified, if an ordinary convex lens is used, the corners will become dark and blurred phenomenon. This is because the refraction of light only occurs at the interface of the medium. The convex lens sheet is thicker, and the part where the light propagates in a straight line in the glass will attenuate the light. If you can remove the part that propagates in a straight line and only keep the curved surface where refraction occurs, It can save a lot of material while achieving the same light-gathering effect. The Fresnel lens uses this principle. The Fresnel lens looks like a piece of glass with countless concentric circular patterns (ie Fresnel bands). But it can achieve the effect of a convex lens. If the projected light source is parallel light, the brightness of the image can be kept consistent after convergence and projection, so I use a Fresnel lens as the lens of the projector.
