That’s what a so called diffraction grating will do. Mine has 1000 lines per mm. Basically, when light hits this "transparent film", it will split the light into all its colors. As you can see in the video, using white light, we can see the full spectrum from red to blue. So basically, when light hits this film, it will point the red color more to the left, the green light somewhere in the middle and the blue color more to the right. Once we have the color spectrum we have to measure the intensity of the colors. For that I have a digital camera with the TSL1401 chip. I take the lens off and as you can see, we have that linear sensor. We can read the sensor values with the arduino. The camera has 128 pixels and we can get the intensity value of each pixel on the A0 pin and we can measure that with the Arduino analog inputs. All we have to do is to make the color spectrum hit the entire width of the camera sensor. Then we use the Arduino MEGA to read each pixel of the camera and display the values on the TFT display.