I thought I'd jump in here on a Sunday (with NASCAR playing on the plasma in the background) because for filmmakers, the integrity and design of this single block can not be overemphasized. This is a hugely important point of discussion that needs to be understood if we are to be successful at getting what's in our heads down onto the screen. In fact, we'll be having an optical engineer discuss just this process, with translation to a language we all understand thankfull, next month on
www.2ndUnit.tv .
I know I'll probably get argument if for no other reason than there exist myriad areas that are important but, for me, the integrity of this one area of the camera and the work that goes into it is more responsible for the look of our work product than any other single operation the camera performs. And, I believe this because it is here that the image splits up into its red, green and blue components, each of which is then directed to to a receptor dedicated to that color which is the very foundation of video image recordation. From there the colors travel to the chip(s) with its (their) hundreds of thousands and millions of sensors to be processed into an image or series of images. Thus the integrity of the signal that enters the sensor contributes greatly to the quality of the images that exit the sensors on their way to being combined into a picture; the proverbial GIGO.
As an image, with its three basics colors, enters the lens, it is ony as good as the quality of the atmosphere through which it must travel to get to the lens. Simply stated, the smorrier the day in LA, the mnore deggraded the image arriving at and travelling through the lens. The same principle applies inside the camera with the image block and it is here where cheaper cameras make themselves known becuase the pathway inside the splitter is the key to a quality signal which is then sent to the sensors.
As the image travels through the lens, it encounters three prisms that are glued together between which are two dichroic mirrors. These mirrors are glass with coatings which are sensitive to particular wavelenghts and thus energy measurements of individual light waves. When presented with three colors, the mirrors reflect one color with an individual "color signature" while allowing the others to pass through unimpeded. Thus, the quality of the reflective coating and the quality of the glass in the mirrors and prisms are the first important factors of the image splitter block. The second all-important factor is the quality of the environment or the "air" in which the light travels.
So, an image passes through the lens and enters the "beam splitter" which contains the prisms and mirrors. The first mirror is red-sensitive and reflects the red off to a red sensor while allowing the green and blue to continue on through impeded only by the imperfections in the coating and the glass and, again, the atmosphere in which it is travelling. As the light reaches the second mirror, blue is reflected off to the blue receptor by the second mirror whose coating is blue-selective and the green continues on, again unimpeded by anything other than the coating, the glass and the atmosphere in which it s travelling. The green finally reaches its own receptor. As an aside, originally, filters were used to separate the colors but it was found that mirrors maintain the luminance integrity of the signal better.
It's important to note that each prism varies in thickness which slows the procession of the signals. The reason for this is that each color needs to arive at the "taking lens" where the recording process begins at exactly the same time. With mirrors changing the speed and distance each R,B and G signal travels, varying the thickness of the mirrors is the only way to control the arrival times of the beams. Thus, with the green light beam travelling a shorter, straight path to its receptor and the red and blue being reflected off taking circuitous routes to their receptors, without increasing the thickness and thus the impedence to which the green signal is subjected before meeting the green receptor, the green would reach the receptor well before the red and the blue and thus arrive at the taking lens before the red and blue. This would be wholly contrary to the requirement that the R,G and B signals arrive at the taking lens concurrently. As another aside, I'm not sure if it goes in exactly this sequence but red, green and blue simply match the RGB moniker.)
You can see now how the quality of the coatings and glass affect the quality of the signals that they direct but with speed and distance controlled, the only other variable that affectes the quality and timing of the signal and thus the image is the atmospheric and environmental conditions in which the light beam, made up of energy, travels. An image travelling through anything other than an absolutely pristine environment such as that found in a vaccuum results in signal denigration via energy bleed-off and this is where flare comes from. (Think back to the adverse effect smog can ahve on an image before it hits the lens in Los Angeles!) Particulate matter in the "vaccuum" and other forms of pollution serve to drain the energy off en-route to the sensor resulting in a degraded beam reaching the sensors. Thus, the quality of the images coming from a camera is first determined by the quality of the image spitting block about which you inquire. To get the highest possible image requires the employing the highest quality glass and coatings available and then positioning them precisely within as pure a vaccuum environment as is possible. Now here's the kicker; irrespective of the quality and care taken, the process, inandof itself, as good as it is, still degrades the original image and this is where advanced circuitry come is which attempts to restore the lost quality but that's a simple story for another time.
Oh and the Fed Ex #11 car, with Denny Hamlin a rookie inside, just won today's NASCAR race. Score one for the independent filmmakers and rookies everywhere!!!
