Module 2 Week 2 Pre-lab
Last week you determined the spacing of a diffraction grating by shining a red laser through it and measuring the distances \(L\) from grating to viewing screen required to make the first diffraction maximum appear at certain distances \(x\) from the central bright spot. In that investigation, you became very familiar with diffraction from a one-dimensional pattern of barriers and apertures (the grating slits).
The theme of this week’s session is two-dimensional grating patterns, of which pixel arrays on a projector or phone screen are fun examples. You will observe the diffraction pattern created by two gratings oriented perpendicular to each other, and then use techniques from last week to measure the spacing of pixels on an LCD panel we have pulled from an old projector. You may have time at the end to explore what it would take to measure your phone screen’s pixel pattern via a similar method.
Background
A liquid crystal display (LCD) consists of a two-dimensional grid of small boxes called pixels (originally short for “picture elements”) that, together, display an image on a screen. Each pixel is itself composed of three ‘sub-pixels’ with colors red, green, and blue. If you look at your computer screen through a camera, you might be able to see the pixelated nature of the screen. Some common arrangements of pixels and sub-pixels in different devices are shown in the figure below. Voltages are applied across a pixel to control the colors and their relative intensities in that pixel.
Figure 1 — LCD sub-pixel layouts for various types of screens.
You can think of the panel as a grid of windows with either red, green or blue glass, illuminated by a bright light from behind. Some mechanism – perhaps a person adjusting the window shade, or (actually) a series of polarizers – determines how much light passes through each individual window. If you stand far enough away, an adjacent set of red, green, and blue windows is too small for you to see each color separately; instead you see a splotch of color made up of whatever red, green, and blue light is getting through that set of windows. Again, if you stand far enough away, the individual splotches blend together and you see a color picture made up of dots the size of each individual window. Projector displays, your computer screen, and any LCD screen –such as the one on your phone– for that matter, operate like this with “windows” or pixels on the order of tens to hundreds of micrometers wide. The more pixels a panel has, the sharper the image it produces, since there is more room for ‘perfecting’ the image on small scales.
If we shine a red laser through an unpowered LCD panel, the red light can transmit only through the red sub-pixels, of which there is one per pixel overall. Thus, you can think of the LCD as a two-dimensional grid of apertures, where the distance between the apertures is the center-to-center pixel spacing.