Introduction

This is not intended to be a full-fledged thesis on color space nor is it intended to be a highly technical paper, although it will be necessary to present several definitions and will touch on areas that can be a bit confusing. I hope that this quick study will help in presenting your images, alleviate some of this confusion and help you set up a work flow to produce the images you want. It won’t help you take better images – only produce images in the proper color space so they display or print correctly.

At the end of the paper is a list of references.

Background

There has never been a time in history with such diversity in image-capturing devices as there is today, from cell phone camera to highly sophisticated large-format digital cameras and everything in between. There has never been such capabilities in capturing an image. The good news is that the technology keeps improving and expanding. The bad news is that we have to try and keep up with it.

Rather than talk about any particular camera, I will talk about the basic underlying mechanics that are common to the majority of these devices.

All digital cameras use an electronic sensor to receive the light reflected from our subject and then convert it into digital information. This sensor can be thought of as a replacement for film, but only in a crude sort of manner. With film, you selected a particular film to achieve a specific result.e.g. Color, black & white, fine grain, infrared, etc. But you can’t change your sensor the way you changed film, so you need to capture as much information as possible and then process the data using a computer program to produce the desired image. You can think of it as digital development or as a digital darkroom. The digital information that has been captured is then stored in a file using a particular format.

File Formats

The digital information that makes up the image has to be stored in some sort of common format that will allow a number of different programs to work on it as well as display it.

In the beginning, there were a number of different storage formats. There still are, but there has been progress in settling on a couple common formats which does help make our lives a little easier.

What is RAW

A camera raw image file contains minimally processed data from the image sensor of a digital camera. Raw files are so named because they are not yet processed and therefore are not ready to be printed or edited with a bitmap graphics editor such as Adobe’s Photoshop. Normally, the image is processed by a raw converter in a wide-gamut internal color space where precise adjustments can be made before conversion to a file format such as TIFF or JPEG for storage, printing, or further manipulation. These images are often described as “RAW image files”, although there is not actually one single raw file format. In fact there are a large number of such formats in use by different models of digital equipment (like cameras or film scanners).

Raw image files are sometimes called digital negatives, as they fulfill the same role as negatives in film photography: that is, the negative is not directly usable as an image, but has all of the information needed to create an image. Likewise, the process of converting a raw image file into a viewable format is sometimes called developing a raw image, by analogy with the film development process used to convert photographic film into viewable prints. The selection of the final choice of image rendering is part of the process of white balancing and color grading.

Like a photographic negative, a raw digital image may have a wider dynamic range or color gamut than the eventual final image format, and it preserves most of the information of the captured image. The purpose of raw image formats is to save, with minimum loss of information, data obtained from the sensor, and the conditions surrounding the capturing of the image (the meta-data).

Some example RAW formats:

.3fr (Hasselblad)
.arw .srf .sr2 (Sony)
.bay (Casio)
.crw .cr2 (Canon)
.cap .iiq .eip (Phase_One)
.dcs .dcr .drf .k25 .kdc (Kodak)
.dng (Adobe)
.erf (Epson)
.mef (Mamiya)
.mos (Leaf)
.mrw (Minolta)
.nef .nrw (Nikon)
.orf (Olympus)
.ptx .pef (Pentax)
.raf (Fuji)
.raw .rw2 (Panasonic)
.raw .rwl .dng (Leica)
.x3f (Sigma)

What is JPEG

The Joint Photographic Experts Group (JPEG, or JPG) standard is a commonly used method of lossy compression for photographic images. Lossy means that information is permanently lost or discarded during the conversion process. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. This is what makes it so popular among camera makers. Also. It is an established standard.

JPEG compression is used in a number of image file formats. JPEG/EXIF is the most common image format used by digital cameras and other photographic image capture devices. It is the most common format for storing and transmitting photographic images on the World Wide Web.

The JPEG file format is best for photographs and scenes with smooth variations of tone and color. For web usage, where the image file size is important, JPEG is very popular.

As a side note, JPEG may not be as well suited for line drawings and other textual or iconic graphics, where the sharp contrasts between adjacent pixels can cause noticeable artifacts caused by the compression. This would also hold true for images that have lots of straight lines and edges. Generally, you would use a lose-less method such as TIFF or PNG or even use a vector-based graphic format such as SVG.

JPEG is not well suited to files that will be edited multiple times, as some image quality will be lost each time the image is decompressed and recompressed, particularly if the image is cropped or shifted, or if encoding parameters are changed. To avoid this, an image that is being modified on a regular basis can be saved in a lose-less format, with a copy exported as JPEG for final distribution.

Other formats (TIF,PNG,etc.)

If you need to save an image without losing information, then you would want a lose-less format such as TIF ort PNG.

Tagged Image File Format (abbreviated TIFF or TIF) was originally created as an attempt to get desktop scanner vendors of the mid-1980s to agree on a common scanned image file format, rather than have each company promote its own proprietary format. In the beginning, TIFF was only a binary image format (only two possible values for each pixel), since that was all that desktop scanners could handle. As scanners became more powerful, and as desktop computer disk space became more plentiful, TIFF grew to accommodate gray scale images, then color
images. Today, TIFF is a popular format for high color-depth images, along with JPEG and PNG.

Portable NetworkGraphics PNG is a bitmapped image format that employs lossless data compression. PNG was created to improve upon and replace GIF (Graphics Interchange Format) as an image file format not requiring a patent license.

PNG supports a standard set or colors or palette-based images as well as greyscale images. It is the only cross-platform bitmap format that supports transparency (an alpha channel). The PNG format was designed for transferring images on the Internet, not for print graphics. It is not known to be used in any camera.

The PSD (Photoshop Document), Photoshop’s native format, stores an image with support for most imaging options available in Photoshop. These include layers with masks, color spaces, ICC profiles, transparency, text, alpha channels and spot colors, clipping paths, and duotone settings.

Photoshop’s popularity means that the PSD format is widely used, and it is supported to some extent by most competing software. The PSD file format can be exported to and from Adobe Illustrator, Adobe Premiere Pro, and After Effects, to make professional-standard DVDs and provide non-linear editing and special effects services, such as backgrounds, textures, and so on, for television, film, and the Web. Photoshop is a pixel-based image editor, unlike programs such as Macromedia FreeHand (now defunct), Adobe Illustrator, Inkscape or CorelDraw, which are vector-based image editors.

Photoshop uses color models RGB, lab, CYMK, greyscale, binary bitmap, and duotone. Photoshop has the ability to read and write raster and vector image formats such as .EPS, .PNG, .GIF, .JPEG, and Adobe Fireworks.

Color Model and Color Space

The following is an extract from Wikipedia: A color model is an abstract mathematical model describing the way colors can be represented as an ordered list of numbers, typically as three or four values or color components .e.g. Red, Green, Blue (RGB), and Cyan, Magenta, Yellow & Black (CMYK) are color models. This basically means that each color has a numeric value, generally expressed as a hexadecimal (base 16) value assigned to it.

Yeah, quite a mouthful. Basically, there are two color models that we are concerned with primarily RGB and secondarily CYMK. I will address the CYMK color space in another article and compare RGB and CYMK.

Another extract from Wikipedia: Adding a certain mapping function between the color model and a certain reference color space results in a definite “footprint” within the reference color space. This “footprint” is known as a gamut, and, in combination with the color model, defines a color space. For example, Adobe RGB and sRGB are two different absolute color spaces, both based on the RGB model.

Basically, the gamut is the overall size of the color space and has a reference point for the color white, from which all other colors are referenced. Generally this is the white point.

Gamut (from Wikipedia)

In color reproduction, including computer graphics and photography, the gamut, or color gamut, is a certain complete subset of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a certain output device. Often it refers to the complete set of colors found within an image at a given time. When converting a digital image to a different color space, or outputting it to a particular output device generally alters its gamut, in that some of the original colors are lost in the process.

White Point (from Wikipedia)

A white point (often referred to as reference white or target white in technical documents) is a set of numbers that define the color “white” in image capture, encoding, or reproduction. Depending on the application, different definitions of white are needed to give acceptable results. For example, photographs taken indoors may be lit by incandescent lights, which are relatively orange compared to daylight. Defining “white” as daylight will give unacceptable results when attempting to color correct a photograph taken with incandescent lighting.

LAB

The LAB color space is kind of a theoretical thing that’s kind of hard to wrap your mind around.

Here is the definition from Wikipedia: A Lab color space is a color-opponent space with dimension L for lightness and a (Red-Green) and b(Blue-Green) for the color-opponent dimensions, based on an international standard established by CIE. (CIE XYZ color space coordinates)

Unlike the RGB and CMYK color models, Lab color is designed to approximate human vision. It tries to present a uniform color perception with the L component basically adjusting lightness. From a human eye perspective, you can make some pretty good color adjustment by adjusting these three components in RGB or CMYK spaces, which are designed around a device rather than the human eye.

Because Lab space is much larger than the gamut of computer displays, printers, or even human vision, a LAB bitmap image requires more data per pixel to obtain the same precision as an RGB or CMYK bitmap – in other words, a bigger file. This goes back to the 1990s, when computer hardware and software was mostly limited to storing and manipulating information in 8 bit/channel bitmaps. Converting an RGB image to Lab and back was a lossy operation. Today with the ability to work with 16-, 24- and 32- bits/channel this is not as much a problem as it was.

Additionally, many of the “colors” within Lab space fall outside the gamut of human vision. These “colors” cannot be reproduced in a manner that we can ‘see’ them with our eyes. These would basically include infrared and ultraviolet. However, built-in color management software in image editing applications, such as PhotoShop, will pick the closest in-gamut approximation, changing lightness, colorfulness, and sometimes hue in the process.

Bottom line, LAB is not used a lot outside of Photoshop. This portion of the paper is only included to provide you with a little bit better understanding of color space and what it involves.

RGB

The RGB color model is an additive color model in which red, green, and blue light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three additive primary colors, red, green, and blue.

The main purpose of the RGB color model is for the sensing, representation, and display of images in electronic systems, such as televisions and computers, though it has also been used in conventional photography. Before the electronic age, the RGB color model already had a solid theory behind it, based on human perception of colors.

RGB is a device-dependent color model: different devices detect or reproduce a given RGB value differently, since the color elements (such as phosphors or dyes) and their response to the individual R, G, and B levels vary from manufacturer to manufacturer, even in the same device over time. Thus an RGB value does not define the same color across devices without some kind of color management.

Typical RGB input devices are color TV and video cameras, image scanners, and digital cameras. Typical RGB output devices are TV sets of various technologies (CRT, LCD, plasma, etc.), computer and mobile phone displays, video projectors, multicolor LED displays, and large screens such as JumboTron, etc. Color printers, on the other hand, are not RGB devices, but subtractive color devices (typically CMYK color model). While we may send an RGB image to the printer, if the printer is properly set up, it will convert the RGB image to CYMK for printing.

Another Wikipedia definition: RGB color space is any additive color space based on the RGB color model. A particular RGB color space is defined by the three chromaticities (the quality of a color or light with reference to its purity and its dominant wavelength, not its intensity or brightness) of the red, green, and blue additive primaries, and can produce any chromaticity that is the triangle defined by those primary colors. The complete specification of an RGB color space also requires a white point chromaticity and a gamma correction curve.

In other words it is a color space based on RGB with a known reference point or white point.

Gamma

Gamma correction, gamma nonlinearity, gamma encoding, or often simply gamma, is the name of a nonlinear operation used to code and decode luminance in video or still image systems. Think in terms of a brightness that changes value along curve rather than a straight line.

In most of today’s computer systems, images are encoded with a gamma of about 0.45 and decoded with a gamma of 2.2. In older Macintosh computers, prior to Mac OS X 10.6 (Snow Leopard) the gamma was 0.55 and 1.8 respectively. In any case, data in digital image files (such as JPEG) are encoded with a predefined gamma value. Applications, such as Adobe Photoshop will optionally allow you to select which gamma value ( 1.8 or 2.2) to decode the image with. Additionally, printers such as the Epson R2400 will allow you to specify which gamma value to print with.

The sRGB color space standard used with most cameras, PCs, and printers has a decoding gamma value near 2.2 over much of its range.

sRGB

sRGB is a standard RGB color space created cooperatively by HP and Microsoft in 1996 for use on monitors, printers, and the Internet.

The sRGB color space has been endorsed by the W3C, Exif, Intel, Pantone, Corel, and many other industry players; it is used in proprietary and open graphics file formats, such as SVG.

The sRGB color space is well specified and is designed to match typical home and office viewing conditions, rather than the darker environment typically used for commercial color matching.

As the recommended color space for the Internet, sRGB should be used for editing and saving all images intended for publication to the
world wide web.

Due to the standardization of sRGB on the Internet, on computers, and on printers, many low- to medium-end consumer digital cameras and scanners use sRGB as the default (or only available) working color space. As the sRGB gamut meets or exceeds the gamut of an inkjet printer, an sRGB image is often regarded as satisfactory for home use. However, consumer-level CCDs are typically uncalibrated, meaning that even though the image is being labeled as sRGB, one can’t conclude that the image is color-accurate sRGB.

If the color space of an image is unknown and it is an 8- to 16-bit image format, it is safe to assume it is using the sRGB color space. This allows a program to identify a color space for all images, which may be much easier and reliable than trying to track the “unknown” color space. An ICC profile may be used, the ICC distributes several such profiles. (The discussion of ICC profiles is a bit outside the scope of this article and could very well be another article on its own. In the meantime, if you want to learn more about ICC profiles you could start at Wikipedia.)

Images intended for professional printing via a fully color-managed work flow, e.g. prepress output, sometimes use another color space such as Adobe RGB (1998), which allows for a wider gamut. If such images are to be used on the Internet they may be converted to sRGB using color management
tools that are usually included with software that works in these other color spaces.

Adobe RGB(1998)

The Adobe RGB color space is an RGB color space developed by Adobe Systems in 1998. It was designed to encompass most of the colors achievable on CMYK color printers, but by using RGB primary colors on a device such as the computer display. The Adobe RGB color space encompasses roughly 50 percent of the visible colors specified by the Lab color space, improving upon the gamut of the sRGB color space primarily in cyan-greens. Generally, when images using Adobe RGB are projected or used on a web page, they will appear dull/flat/washed out. That is because most display and projecting devices are calibrated to sRGB standards.

Wide Gamut RGB

The Wide Gamut RGB color space is an RGB color space developed by Adobe Systems that offers a larger gamut by using pure spectral primary colors. It is able to store a wider range of color values than sRGB or Adobe RGB color spaces. As a comparison, the Wide Gamut RGB color space encompasses 77.6 percent of the visible colors specified by the LAB color space, whilst the standard Adobe RGB color space covers just 50.6 percent and sRGB covers only 35 percent.

When working in color spaces with such a large gamut, it is recommended to work in a minimum of 16-bits per channel color depth to avoid posterization effects. Posterization will occur more frequently in 8-bits per channel modes as the gradient steps are much larger.

As with sRGB, the color component values in Wide Gamut RGB are not proportional to the luminance. Similar to Adobe RGB, a gamma of 2.2 is assumed.

ProPhoto RGB

The ProPhoto RGB color space, also known as ROMM RGB (Reference Output Medium Metric), is RGB color space developed by Kodak. It offers an especially large gamut designed for use with photographic output in mind. The ProPhoto RGB color space encompasses over 90 percent of possible surface colors in the LAB color space, and 100 percent of likely occurring real world surface colors making ProPhoto even larger than the Wide Gamut RGB color space. One of the downsides to this color space is that approximately 13 percent of the representable colors are outside the range of perception of the human eye and are not visible colors. These would include infrared and ultraviolet. This means that potential color accuracy is wasted for reserving these unnecessary colors.

When working in color spaces with such a large gamut, it is recommended to work in a minimum 16-bit color depth to avoid posterization effects. This will occur more frequently in 8-bit modes as the gradient steps are much larger.

Summary

Camera settings for color space can be critical when capturing TIF or JPG images. For RAW image capture, it makes little difference what color space you select, since you can set it just effectively in your editing software.

A somewhat simplistic example might be to think of the various color space as being a number of different sized containers:

RAW would be something like a pool not having a specific size. Pools come in all sorts of sizes, some larger than others, some smaller – just like the RAW file format.

Let’s say that LAB is a gallon-size container, probably the largest container that we really want to work with. It holds the lump sum total of the known colors that we can see, plus some that we can’t.

ProPhoto RGB might be a ¾ gallon container. It holds a lot of colors, but not as much as LAB.

The Adobe RGB might be a ½ gallon container which holds quite a bit, but less than the previous containers. It is also a convenient size for working with.

sRGB is the quart size. Convenient, easy to hold and work with.

Now that we have a size reference established we can start working with the various color spaces. In order to work with an image, you need to gather the colors into a manageable size or color space. This process is RAW conversion.

So we process our pool-sized RAW color space into something that we can carry home. It doesn’t have the total pool, just some essential information. We probably don’t want a LAB size container, but more likely a ProPhoto or Adobe RGB size container. These are more convenient sizes to work with.

Now, if you want to move an image from a larger color space to a smaller color space, you will need to distill it down to the essential colors that will fit in the smaller space. Otherwise, if you just try and pour the larger container into the smaller container, some of the color information just overflow and get lost. Your image will not look right. Keep in mind that this is a one-way trip. Once you convert an image from a larger color space to a smaller you can not recapture the information that you removed in order to make the image fit into the new smaller space. It is sort of like taking the quart-sized container and dumping the contents into the ½-gallon container. Yes it fits, but it is not full.

As a suggestion, I would process a RAW image file into a ProPhoto File in Lightroom/PhotoShop and do whatever work you need to do. Then export it to a sRGB JPG file, if you are going to put the image on the web or use a digital projector for showing it.

If you are going to print the image, particularly on an Epson printer, I’d export the image as an Adobe RGB image and print it. (The Epson printer as this is the only printer that I have firsthand experience with.)

I hope that this helps in understanding the mystery of color space.

Below is a graphic representation that came from Wikipedia