Lighting – Color

Lighting in Digital Photography
Colors of Light Source and of Pictures

This article highlights some basic aspects related to color in digital photography for improving the knowledge of photography fans. We put together into an organized form several aspects very well known in digital color photography with links to relevant Internet sites when clarification requires more space. There are probably at least several thousands articles related to this subject on Internet and not only. We hope that this article will rise interest for further reading on Internet and elsewhere and will also nurture some useful discussions.

The Digital Picture Flow Chart schematic summarizes this article. Digital picture is a two-step process as it is any classical picture: shoot the picture and see the picture.

Step 1. Shoot the picture

We consider a simple case of an operator shooting a scene illuminated by sunlight at noon. The scene reflects the sunlight toward the operator’s eyes and also toward his/her digital camera, situation labelled A in Digital Picture Flow Chart schematic.

The operator sees the color scene with his/her eyes and his/her brain. This combination is by far better than any existing digital camera, because is based on human experience and thinking.

The digital camera uses a lens that makes the image of the scene on its color image sensor. We correctly assume only geometric distortion introduced by the lens and no color distortion. The color perception of the image sensor is based on additive RGB color model, with Red, Green and Blue as primary colors. According to this model, the white light from any source can be synthesized from the primary colors, as shown in Additive Primary Colors diagram.

The schematic sRGB Color Triangle shows the loci of primary RGB colors or wavelengths and their relative intensities as defined by the additive color model. The point D65 is considered pure white as the outcome of summing theoretical primary colors according to a relationship between their intensities defined by the additive color model. In reality, theoretical primary colors are very difficult or impossible to get at the wavelengths defined by the additive RGB model and at reasonable price for large dissemination of color light emitting devices. Instead, there are used other three available RGB colors close to primary colors. By consequence, with available colors the addition outcome is not pure white. However, there is a quite narrow region around D65 point where white is acceptable with faint tones, usually either toward yellow or blue. Accordingly, all colors across the entire picture are changed. The generic color sensor of digital camera is shown in the schematic below.

The color sensor has a mosaic-type color filter layer on top of photo sensitive elements or pixels for making three monochrome replicas in Red, Green and Blue of color image incident on image sensor surface. Each monochrome replica has smaller number of pixels than the incident color image, but the total number of pixels of all three Red, Green and Blue images is equal with the number pixels of the color image. In other words, the mosaic color filter re-samples the incident color image in separate Red, Green and Blue images with lower spatial resolutions. It is very important to mention several aspects of mosaic color filter:

(i) The mosaic filter has Bayer filter configuration based on additive color model, which also mimics the light perception of human eye. In Bayer mosaic filter, from the entire image sensor area Green elements cover 50%, Red and Blue elements cover 25% each. This article will consider only Bayer mosaic filter. There are in use now several variations of Bayer filter with different colors and their arrangements, but considering them, too, is far beyond the scope of this article. Those interested in this subject can look further on Internet or somewhere else for more information.

(ii) Each element of mosaic filter overlaps perfectly only one pixel. It is obvious that the color information is encoded across the entire area of the image sensor.

(iii) We assume for now the digital camera white balance selection in SUN position. When the operator presses the shutter button, the image sensor makes inside it an electronic image, which is the replica of the optical image on its surface made by the lens and filtered by the mosaic filter. Obviously, the electronic image appears during the exposure time. Setting the duration of exposure time will be the subject of another article.

(iv) The digital image controller reads the electronic image from sensor and decodes the information from pixels using demosaicing algorithms for obtaining the entire color picture in RAW format which contains maximum information about the picture. This format is not yet readable by any display device or printer. RAW format is the electronic equivalent of negative in film photography. Using their proprietary software, most manufacturers of digital SLR cameras make available RAW format to users for further processing on computer. Usually, professional photographers and high end amateurs use RAW format and additional software applications to process images starting from RAW format and saving their work in widely accepted .jpg, .jpeg, or .jpe compressed format. All digital cameras process further the RAW format for delivering pictures in very familiar .jpg, .jpeg, or .jpe format.
We kept this description of converting the optical color image in electronic file as short as possible, to not harass the reader with too many details. For those interested on this subject, we recommend further reading on Internet or somewhere else. However, even from this simple and short explanation, you realize the multitude of sources of errors only during the process starting with the optical image of the scene incident on image sensor and ending with the “trivial” .jpg file.

(v) Further, .jpg file is used by vast majority of display devices such as LCD rear camera monitors, LCD computer monitors, by printers and in certain conditions by LCD TVs, plasma TVs and digital projectors.

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Step 2. See the picture

Obviously, the operator takes a look at the scene before shooting it and remembers the scene. After shooting the picture, the operator looks at the picture of the scene displayed on several devices labelled in Digital Picture Flow Chart schematic such as:
(i) rear camera LCD, labelled B
(ii) LCD computer monitor or TV, labelled C
(iii) on-screen projection from digital projector, labelled D
(iv) printed scene on photographic paper, labelled E.

Both display devices B and C use LCD technology which will be reviewed briefly using the schematics below. A careful look at any LCD screen reveals vertical color stripes such as in Color LCD Pixels Pattern schematic. The encircled area is a color pixel. When zooming it as in Zoom LCD Color Pixels schematic, we see the R,G,B sub-pixels structure used to generate the multitude of colors of the color pixel according to additive color model. The color pixels are used to build the entire pattern of color stripes of LCD screen.

The schematics above shows color LCD from the front side or viewer side. The LCD is illuminated from the backside with light emitted by a white source such as LED, Electroluminescent panel (ELP), Cold Cathode Fluorescent Lamps (CCFLs), Hot Cathode Fluorescent Lamps (HCFLs), External Electrode Fluorescent Lamps (EEFLs). LED-based backlights color LCDs come in two options: either several white LED illuminating the entire screen, or each sub-pixel is a single color LED.
IPS technology uses transistors for individual addressing of transparent sub-pixels working as light valves. Both the transistors and the filters are built on a thin film transparent layer attached to the screen. Each sub-pixel is also a band pass filter for respective Red, Green and Blue primary colors according to RGB additive model. Practically, each of these colors are slightly off the required wavelengths, therefore the resulting white is not pure. It is obvious that the white of the highlighted color pixel in Zoom LCD Color Pixels schematic depends on the colors of back illumination source and also of the colors of Red, Green, and Blue sub-pixels. This color is located somewhere as close as possible to D65 point. There is a tremendous effort of display manufacturers for generating the white color as close as possible to D65 point, thus achieving the best color purity.
In summary, assuming that the LCD color monitor has a perfect color signal at its input, the displayed image depends strongly on the LCD monitor or LCD TV.

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Sometimes you can display on Plasma TV your video clips and movie productions. On the Digital Picture Flow Chart schematics, plasma TV known also as Plasma Display Panel TV or PDP TV is on the same C category as light emitters, such as LCD TV.

PDP TV has the same color pixel and sub-pixel structures as shown in Color LCD Pixels Pattern and in Zoom LCD Color Pixels schematics. The major difference is in generation of R,G,B primary colors. Plasma Display Panel Schematic shows the basic elements for generating each R,G,B sub-pixel. The entire PDP consists of an enclosure filled with neon at low pressure and a matrix of address electrodes and display electrodes shown in schematic as yellow stripes. The R,G,B color sub-pixels containing the respective color R,G,B phosphors are located at the overlapping areas of address and display electrodes. A single specific color pixel is activated by addressing simultaneously one display electrode and all three R, G and B address electrodes behind the sub-pixels of that specific color pixel. A glow gas discharge appears between the display electrode and each of R, G and B address electrodes and accordingly, the Red, Green and Blue phosphors are emitting light with intensity dependent on the voltage between the overlapping electrodes. You noticed that pixels of PDP are light emitters, not band pass filters as in most LCD panels. It is easy to notice that color matching issue between R,G,B colors of PDP and primary colors of additive RGB color model still remains.

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At this point, some comments on pixel size of digital cameras and displays are welcome.
As you notice in Color Image Sensor Schematic, the image sensor is an array of individual sensors or pixels which take as many samples from the continuous image on it, as many pixels are. Rendering image details in digital photography and video is related to center-to-center pixel spacing. Smaller pixels spacing gives more details of picture made by digital camera or by video camera. In other words, if two image sensors have the same size, the sensor with more pixels gives an image with more details. Commercially, digital cameras are advertised by the pixel count of the image sensor, often not mentioning the sensor size. A digital SLR camera with 18MP and a digital point-and-shoot camera with 16MP have good rating at the time of writing this article. For the same reason, the pixel count is used also to advertise camcorders. A good camcorder has a pixel count of 8.9MP or higher.

The situation is different with computer monitors and digital TV sets. Actual models of computer monitors and HDTV sets should be compatible with both 4:3 and 16:9 aspect ratios. HDTV sets have three options: 1080p, 1080i or 720p. Regardless of screen size, the number of pixels is the same in each category. Therefore, the pixel size depends on screen size. However, HDTV has maximum 2MP on its screen. A medium-size, good quality picture from a digital camera has 3456×2304 pixels, which must fit in maximum 1920×1080 pixels. When you see the image on computer monitor, the computer does picture interpolation to fit the screen. When you connect the digital camera directly to TV, the camera does the interpolation. Of course, during interpolations fine details of the image are lost and the colors are degraded, too.

Retina Display of Apple iPads have higher pixel count than required by HDTV requirements, for pleasant appearance. By consequence, Retina Display does not appear pixelated if you look at it from a normal distance. However, even Retina Display does not have enough pixels to match the number of pixels of medium-size good quality digital picture, and the interpolation is required. All the other color-related aspects discussed above apply also to Retina Display.

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As partial conclusion, the operator which shot the picture and still remembers it, sees the color differences between the object and its picture displayed either on rear camera LCD, on LCD computer monitor or on HDTV.

Digital projector is another option to see the digital pictures, labelled D in Digital Picture Flow Chart. These devices are also emitters producing the colors according to additive RGB color model. We will summarize here some basic aspects related to colors related to digital projectors. For more information on operation of digital projectors, the reader is invited to follow the link. Commercial digital projectors use three technologies.
a. DLP or digital light processing technology is based on a multitude of mirrors for reflecting the light coming from color LED in most situations. DLPs are used typically for conference room presentations, but they can be used home, too.
b. LCD projector technology uses one LCD light valve matrix for each R,G and B color. Red, respectively Green and Blue LEDs are used as light sources. The color projection on screen follows also the additive RGB color model. Obviously, the color problems are similar with LCD TVs. However, LCD video projectors are used for home and business.
c. Liquid crystal on silicon (LCoS or LCOS) technology uses reflective LCD light valves, a single white light source and RGB filters for primary colors. Commercially available LCoS video projectors have portability advantage.

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All color display devices such as computer monitors, TVs and digital projectors require calibration before use, and eventually periodic adjustments.

We suggest you a simple exercise: Look at the image on laptop screen and also on an external LCD monitor connected to it. You notice the difference, which is normal, even after doing color calibration on each monitor. Based on their previous experience and also on the lack of knowledge of real scene, people ignore small changes of colors and do not pay too much attention to color in white areas. They just take the picture as it is, if the relationships between colors make sense according to their previous experience. Only non-sense colors raise concerns. The human brain operates in this way.

We consider now the situation E of the Digital Picture Flow Chart, when the operator sees the printed picture. This is the particular case of looking at any object which reflects the light. The CMYK color model is used for modeling the human color perception of any light-reflective object. CYMK is a subtractive color model based on the selective reflection of colors by an object illuminated with broad band light, not necessarily with white light. In other words, the object subtracts the reflected colors from the incident light. All other colors of the incident light are absorbed by the object.

CMYK subtractive color model uses Cyan, Magenta, Yellow and Key (black) as primary colors for creating the entire subset of colors or gamut at printing. In the Subtractive Color Mixing schematic you see in the lobes the color outcome of combining primary colors. By combining all three CMY colors such as in the center of the diagram, the outcome is gray, which is non-saturated black. Black is used additionally in printing for enhancing grey tones up to black level. There are two main aspects related to printing: color and sharpness. We comment color first.

Inkjet and laser are dominant color printing technologies on the market.

Inkjet printing, as the name suggests, throws on paper under very strict control, extremely fine droplets of Cyan, Yellow, Magenta and Black ink for printing text and pictures. Digihut.org has more than 1,500 inkjet printers to choose from. Some inkjet printer models such as Epson Artisan use six colors instead of four for delivering more brilliant colors.

Laser printing process is based on the xerographic printing process. This process consists of creating the electrostatic image of the document or picture to be printed on the surface of a rotating drum. A sharply focused laser beam is used to make this image. Extremely fine particles of toner are stuck on the electrostatic image on drum surface, which are further transferred to paper and fixed on it by heating. Digihut.org has more than 5,000 laser printers. Toner thickness on paper gives color saturation.

Natural appearance of picture colors or gray tones and details rendering are the other important feature of printing. Paper smears ink more than it smears toner. In other words, a dot in the image becomes a spot during printing, which can degrade significantly the details of printed document. Printer quality expressed in dots-per-inch or dpi is standard parameter for rating printing details and also for advertising printers for pleasant appearance of printed document. Photo quality printers have 4800dpi or more.

Photo quality paper has special coating on one side with practically no smearing. Photo paper with various finish such as luster, glossy, semi-glossy, matte, give rich colors with the same look and feel as traditional photograph. Ink-jet photo printers are more popular than laser photo printers, based on price-performance trade-off. Accordingly, inkjet printer paper is used more than laser printer paper.

All color printers require calibration before use, and eventually periodic alignment checkup.

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Light Sources

At the beginning of this article we supposed the scene illuminated with sunlight at noon, and we kept this supposition throughout the entire article. Sunlight at noon is considered white light. Multiple situations are encountered in real life, when the scene is illuminated with various light sources. We will review briefly the most relevant cases. The digital SLR cameras have White Balance selection, for informing the digital image controller of the camera about the light source at the time of shooting. The camera controller makes automatic color adjustments according to standard spectrum of the selected light source from an option list such as:
(s1) auto balance
(s2) daylight
(s3) shade
(s4) cloudy
(s5) tungsten lamp
(s6) fluorescent
(s7) flash, either built-in, or recommended by camera manufacturer
(s8) custom (use this for best results if not shooting in sunlight)
(s9) color temperature (for advanced users only)

If White Balance is properly set, your pictures will have amazing colors. The digital cameras are doing this job very well.

WARNING The light sources listed above are given as examples. Check to see what are the selections of your camera.
Always check the White Balance setting before shooting. A badly shot scene cannot be corrected.

Sunlight early in the morning or toward sunset has more red. Accordingly, if the photographer does not take special precaution, the picture will change the entire gamut toward red. Usually, digital SLRs have options to adjust light source color within certain rage departing from factory-defined options shown above. However, the best is custom correction, shooting conditions permitting. Follow the instruction manual of your digital camera. For insufficient light, use speedlite flash either built-in, or as stand alone unit. For stand alone unit, use only the speedlites suggested to work with your camera. Stand alone speedlite make dialogue with the digital camera for delivering only the required light flux for the best exposure. If the digital camera is not able to make the dialogue with the speedlite, most chances are for strong overexposure.

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Conclusion: in real life, the colors of the same elements of the scene are not the same in all display devices! We are always making the best effort to see the pictures on computer monitor, HDTV, digital projector and on photo paper as natural as we can do. However, based on their previous experience, the humans do not notice the small departure of colors from the real ones. The quality of entire picture is evaluated according to its overall appearance.

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