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Understanding Adobe RGB: When a Filename Starts with an Underscore

Color management is a subtle but critical aspect of digital imaging. One often overlooked detail is how some cameras signal the use of wide-gamut color profiles like Adobe RGB using the filename itself. When a filename starts with an underscore _, it indicates that the image was captured using the Adobe RGB color space. This is a filename-based convention and not embedded in the image metadata.

Visual Comparison

Below is a comparison of the same image displayed with and without Adobe RGB interpretation. Notice the richer greens and cyans on the left.

D700 1/250s f/8 ISO 200/24° 8mm f/3,5G

Using Adobe RGB

Without Adobe RGB (as sRGB)

Why Adobe RGB?

Adobe RGB is a color space developed by Adobe Systems in 1998 to encompass most of the colors achievable on CMYK printers. Compared to the standard sRGB, Adobe RGB offers a wider gamut, especially in the green and cyan regions. This makes it ideal for professional printing and high-end image editing workflows.

The CIE 1931 xy Chromaticity Diagram

It shows color hue and saturation (not brightness).

• x-axis: how much red vs green is in the color.
• y-axis: how much green vs blue is in the color.

It comes from the CIE XYZ color system, where Y relates to brightness.
Like a map of all visible colors:

• The curved edge is pure spectral colors (rainbow).
• Inside the shape are all mixtures of colors.
• White (D65) is near the center.

See github

sRGB: R(0.6400, 0.3300), G(0.3000, 0.6000), B(0.1500, 0.0600)
Adobe RGB: R(0.6400, 0.3300), G(0.2100, 0.7100), B(0.1500, 0.0600)
White point D65: (0.3127, 0.3290)

Working with Adobe RGB Images

When working with Adobe RGB images, especially in everyday scenarios, it is important to consider how they will appear across different devices and platforms.

General Use

For typical viewing and sharing, converting Adobe RGB images to sRGB is recommended. sRGB is the standard color space for the web and for devices that do not support color management, which helps ensure consistent appearance.

Accurate Conversion

Before publishing or sharing images online, ensure they are converted to sRGB to maintain consistent appearance across devices. Software with reliable color management support, such as the cPicture App, interprets embedded ICC¹ profiles and filename-based color space conventions with quiet precision. This helps ensure that colors appear as intended, even in situations where other tools might not get it quite right, often without the user realizing why.