In this post we examine color profiles, which are specific flavors of a color model.
Each color model describes a method to turn colors into numbers. But within each of these color models, there are different ways to calculate the numbers. In order to know the precise color that a numerical value represents, you need to know which method was used to do the calculation. The International Color Consortium (ICC) has standardized these methods as color profiles. This helps to ensure that color is consistent from device to device. Here’s how it works:
You can view profiles in Photoshop, but also directly in Mac and Windows OS. In Windows, select an image and go to Properties>Details. On Mac, select an image, open Preview and go to Tools>Show Inspector.
When an image is made, it must be tagged with a color profile to indicate how the color was encoded. The color profile tag lets the computer know how to decode the color properly. If there is no tag, then the application or device will not know precisely how to interpret the color, and will just have to guess what the numbers mean. Given the number of different color profiles in existence, it’s likely that the program will guess wrong. The figure below shows how the same RGB numbers produce different colors depending on which profile interprets the values.
RGB color values don’t mean one specific thing unless they are tagged with the encoding profile. When decoded with the wrong profile, you get a different result - sometimes a very ugly and obvious difference. Tagging your images with a profile lets the color be interpreted consistently.
Chain of Custody
In order to get the color right on the computer, there has to be a color information “chain of custody” that goes something like this:
- The file must have an indication of what color space was used when it was created – a color profile. You can save this information as a tag that names the profile for commonly used ones, or you can embed the entire color profile in the file.
- A program that opens the file needs to know how to use the color profile to properly interpret the colors for each pixel.
- When an image is displayed on a monitor, the computer’s graphics driver compensates for imperfections in the device by factoring in the monitor’s profile. So, for example, if your monitor makes shadows too blue, this profile can take some blue out of the darks.
- Every link of the chain must be present or it will be broken. Just like, well, a chain.
While there are a lot of profiles to choose from, there are a handful of ones that are most common. Here’s a list of typical color profiles and what they’re useful for.
This profile is designed to mimic “typical” monitor color (in 1999). In most cases, it’s the right choice for image output that will be posted to websites or be sent out to consumer photo labs. It’s also the right profile any time you are sending images to people who don’t understand color management. It does not contain as many colors as the other RGB profiles listed below.
sRGB is typically considered a “delivery” profile and may be applied as files are prepared to be sent or posted to the web.
Adobe RGB (1998)
This is a “wider” profile than sRGB, meaning that it contains more colors. It is useful for both 8 and 16 bit-per-channel images (see Friday’s post). Note that when images saved with the Adobe (1998) color profile are viewed on a monitor that does not use color management, the images will look flat and dark. This makes it a poor choice for posting images to the web for a general audience or sending images to people who don’t understand color management.
Adobe RGB is typically considered to be a “working” profile that is used in image editing. It is also used when delivering images for high-quality reproduction, such as commercial or fine art printing.
This is a profile which is designed as a “working” color space. It has a very wide color gamut, and can describe all colors that can be seen by the human eye (as well as many colors that can’t be distinguished). ProPhoto should only be used for 16-bit images, and in most cases, the file should be converted to a different color space for delivery of final images.
ProPhoto should generally be used only as a working profile. It should only be used as a delivery profile when it has been clearly specified by the recipient.
If you are responsible for creating a CMYK conversion for commercial printing, then GRACoL is usually the best choice unless the printer specifies another or sends you a custom profile to use.
Gray Gamma 2.2
When working with grayscale images, use a wide gamma for your editing profile. Gray Gamma 2.2 is a good generic choice.
Matrix profiles vs. LUT profiles
Some color profiles are based on mathematical formulas. These profiles are typically standardized ones like the ones listed above. Because these are standard profiles, it’s usually sufficient to tag the file with the profile name. ICC compliant software will typically already contain standard profiles. However, there’s generally no harm in embedding the profile to be sure it’s available.
There are also profiles that are created by measuring the color response of a device, which then save the results in a table. These profiles are called LUTs, which stands for Look Up Tables. LUT profiles are typically used to characterize specific devices like monitors, scanners and printers.
Since LUT profiles are unique, they must be embedded in order to be used. In most cases, it’s better to convert images with LUT profiles to one of the standard profiles before any type of delivery.
In Friday’s post, we will look at bit depth and how it relates to color reproduction.