ProPhoto to sRGB Conversion with Perceptual Rendering Intent

Relative Colorimetric (RelCol) vs Perceptual Rendering intent (Perceptual)

The two basic methods for absolute color conversion are RelCol and Perceptual.

With RelCol, colors that are out-of-gamut of the destination space are clipped to the gamut boundary of the destination space.

Perceptual rendering does it differently in that colors are compressed by mapping with transformation when conversion takes place from a larger color space to a smaller color space. With proper consideration and implementation, RGB channel clipping and change in hue due to conversion with perceptual rendering can be minimized significantly.

The white point for both sRGB and Adobe RGB (aRGB) are D65, whereas white point for ProPhoto, Beta RGB and some other wide gamut RGB are D50. Photoshop and most graphic applications use CIELab D50 as their internal working color model when doing color computation. When converting to sRGB or aRGB from another absolute color space, it has to be with RelCol, as chromatic adaptation to D50 is required or otherwise white balance will be off. Photoshop ACE (Adobe Conversion Engine) sets conversion to RelCol internally by default when converting to a destination space which is defined with D65. Similarly, when the destination space is D50, ACE forces the conversion with Absolute Colorimetric, as no chromatic adaptation is needed. This is quite difference from conversion to a destination space which employs an output type profile where user has an option for selecting one of the desired rendering intents.

Perceptual Conversion with  PhotoShop action is available for downloading here.