Effect of camera distance and angle on color of diverse skin tone-based standards in smartphone photos

Abstract

Accurate and reproducible color capture is vital in medical photography. Camera distance and angle are particularly important as they are highly variable in a clinical setting. To account for variability in illumination, camera technology, and geometric effects, color standards are often used for color correction. To explore how geometry affects color, we quantified the change in CIELAB color value of a color standard for diverse skin tones at varying smartphone camera distances and angles. Whereas both chromaticity (a* and b*) and lightness (L*) were affected by angle, distance only affected L* (standard error of measurement, SEM > 1 CIELAB unit). Flash usage did not generally reduce distance and angle associated variability. Compared to compressed (JPG) format, raw (DNG) images had decreased median variability across different distances and angles. These findings suggest that in medical photography, inconsistent camera distance and angle can increase variability in photographed skin appearance over time.

Keywords: color calibration; color reference; color standard; skin imaging; teledermatology.

FIGURE 1

(A) Schematic of the experimental setup. Distances and angles between the swatches and each light source were as follows: D = 305 cm and θ = 35° for the soft white light, and D = 225 cm and θ = 45° for the D50 light. D_Exp was held at a constant 1 m for the angle experiments and was varied from 30 cm to 1 m for the distance experiment. (B) Photo of the experimental setup.

FIGURE 2

(A) Setup at 0°. (B) In the parallel (‖) setup, the tilt table is rotated in a way that the top of the table is moving towards the incoming light (towards the bottom of the image). (C) In the perpendicular (⊥) setup, the tilt table is rotated in a way that the top of the table is moving perpendicular to the incoming light (towards the right side of the image). The direction of the light source is indicated by the shadow of the tilt tables.

FIGURE 3

(A) Change in L* value across distances between the camera and the isolated swatches. The line color represents the color of the Pantone swatch. (B) Pantone swatches corresponding to each line.

FIGURE 4

Color variation across distance (A) and angle (B), expressed by standard error of measurement (SEM): comparison of different lighting conditions, rotation angles, and flash usage. Dashed and dotted lines represent the thresholds for fairly noticeable [>5] and just noticeable [>1] differences. The k and ⊥ symbols represent the parallel and perpendicular angle experiments, respectively. SEMs for images with flash off are represented by an empty marker while flash on is represented by a filled marker. Images under D50 light are represented by circular markers while soft white is represented by diamond markers. The 95% confidence interval for each SEM is represented by the error bars. The marker colors represent the color channel used in the calculation, with black representing L*, red representing a*, and blue representing b*.

FIGURE 5

Comparison of DNG and JPG SEMs from all distance and angle experiments. Each point represents a calculated SEM for each imaging condition and each color channel. SEM, standard error of measurement.