Fig 1.
Flowchart of colour quantification based on digital colour image processing for holographic grating sensors.
Fig 2.
Tuning structural colour in a pH-sensitive holographic sensor developed in the Lowe group.
(A) Structural colours in three different pH solutions of 6, 5.25 and 4.75, from left to right; (B) The colour is tuned by controlling the parameter d, the grating spacing and consequently Bragg diffraction from the holographic grating.
Fig 3.
(A) Image acquisition: The captured colour is a function of ambient illuminant (D65 in this study), the replayed colour of the holographic sensor and the spectral sensitivity (characteristics) of the camera (Samsung GT-S5660); (B) An example of captured image of the holographic sensor in the centre of a logo QR code (encoded data: The Lowe Group, Institute of Biotechnology, University of Cambridge); (C) Encrypted and compressed image uploaded to the cloud, followed by decryption and decompression for further image processing; (D) Object recognition in a scene using the SURF method: (D1) Matched the strongest feature points (yellow lines) from the template image in the greyscale to an in-plane rotated image (outlined in green); (D2) Detected QR code with the embedded sensor in the scene (outlined in yellow); (E) Camera characterisation to convert the device-dependent RGB colour values to device-independent CIEXYZ tristimulus values; (F) The corresponding region of interest (ROI); (G) Regression analysis using a multilayer perceptron to derive the analyte of concentration (e.g. pH).
Fig 4.
A sample of captured images from the colour of holographic grating sensor at pH 3.00.
(A) and the corresponding pixel colour distributions in RGB (B), HSI (C) and CIEL*a*b* (D) colour spaces.
Table 1.
The average of the quantitative metric J for three colour spaces of RGB, HSI and CIEL*a*b* for the pair of segmentation method.
Fig 5.
The Bland-Altman plot for the colour coordinates of the CIEL*a*b* colour space for k-means and FCM segmentation methods.
Fig 6.
(A) The CIE1931 chromaticity plot for colour coordinates of the pH-sensitive holographic grating sensors to buffer solutions in the pH range 3.00–6.50; (B) The linear correlation between the colour quantification algorithm and the true pH value of the buffer solutions (2 sensors at 11 pH levels).