Table of Contents

• About the Color Image Database
• Why a New Color Image Database?
• Calibration and Experimental Procedure
• Organization of the Database

About the Color Image Database

The database consists of 130 calibrated color images of natural objects under calibrated illuminations:

• Calibrated images: images are given in CIE XYZ tristimulus values.
• Natural objects: imply complex textures, mutual illumination, and shadows, which induce non-linear effects in the tristimulus values.
• Calibrated illuminations: include diffuse CIE D65 and diffuse CIE A illuminant.

The database is suitable for accurate studies on color image statistics, chromatic adaptation in natural environments, and color constancy.

Why a New Color Image Database?

This color image database was collected in the context of a chromatic adaptation study based on color statistics. The following facts are relevant:

• The proper way to describe the physical input to any (artificial or human) visual system involves either (i) absolute radiances, hyperspectral images, or (ii) absolute tristimulus images.
• Even if the spectral reflectance of objects is known, the simple flat-Lambertian world assumption is not valid due to mutual illumination, shadows, and specular reflections.
• Accurate results on chromatic adaptation, such as corresponding pairs data, require a continuous distribution of samples in the tristimulus space, which necessitates a wide enough ensemble of natural reflectances.

Existing color databases have several limitations:

• Uncalibrated data (digital counts from conventional cameras instead of radiances or tristimulus values).
• Spectroradiometric databases often do not include reflectance but estimate it using a white reference sample, and scenes may not be available under both CIE D65 and CIE A illuminants.
• The above is also true for the available tristimulus image databases.

Given these issues, this calibrated database includes a wide set of natural objects under a pair of calibrated illuminants.

Calibration and Experimental Procedure

The images were taken using a Macbeth Executive light chamber equipped with standard CIE D65 and CIE A illuminants, and a calibrated image colorimeter Lumicam1300. Key details:

• Exposure time for each filter (X, Y, Z) was adjusted to avoid over or under-exposure, ensuring optimal operating range for the camera.
• Pictures with misregistered channels (due to motion) or scenes outside the camera’s operating range were discarded.

Accuracy was verified by capturing 10 hue pages of the Munsell Book of Color and comparing the measured CIE xy chromaticities with theoretical values computed from known reflectances. The accuracy of luminance was roughly within the limits provided by the manufacturer (~3%).

Organization of the Database

The database includes 65 different scenes of natural objects under two illuminants (130 images) plus 10 scenes displaying different hue pages of the Munsell Book of Color (20 images). The image size is 1000x1280 pixels. The images are classified as follows:

Download the complete dataset (all the above files):

Images (MATLAB arrays of size 1000x1280x3) are stored in a MATLAB structure in each of the corresponding .mat files above. Images in the structure are sorted according to the order in the pictures below. Chromatic diagrams with all the colors in each set are also shown below.