The dark art of light measurement: accurate radiometry for low-level light therapy

Abstract

Lasers and light-emitting diodes are used for a range of biomedical applications with many studies reporting their beneficial effects. However, three main concerns exist regarding much of the low-level light therapy (LLLT) or photobiomodulation literature; (1) incomplete, inaccurate and unverified irradiation parameters, (2) miscalculation of 'dose,' and (3) the misuse of appropriate light property terminology. The aim of this systematic review was to assess where, and to what extent, these inadequacies exist and to provide an overview of 'best practice' in light measurement methods and importance of correct light measurement. A review of recent relevant literature was performed in PubMed using the terms LLLT and photobiomodulation (March 2014-March 2015) to investigate the contemporary information available in LLLT and photobiomodulation literature in terms of reporting light properties and irradiation parameters. A total of 74 articles formed the basis of this systematic review. Although most articles reported beneficial effects following LLLT, the majority contained no information in terms of how light was measured (73%) and relied on manufacturer-stated values. For all papers reviewed, missing information for specific light parameters included wavelength (3%), light source type (8%), power (41%), pulse frequency (52%), beam area (40%), irradiance (43%), exposure time (16%), radiant energy (74%) and fluence (16%). Frequent use of incorrect terminology was also observed within the reviewed literature. A poor understanding of photophysics is evident as a significant number of papers neglected to report or misreported important radiometric data. These errors affect repeatability and reliability of studies shared between scientists, manufacturers and clinicians and could degrade efficacy of patient treatments. Researchers need a physicist or appropriately skilled engineer on the team, and manuscript reviewers should reject papers that do not report beam measurement methods and all ten key parameters: wavelength, power, irradiation time, beam area (at the skin or culture surface; this is not necessarily the same size as the aperture), radiant energy, radiant exposure, pulse parameters, number of treatments, interval between treatments and anatomical location. Inclusion of these parameters will improve the information available to compare and contrast study outcomes and improve repeatability, reliability of studies.

Keywords: LLLT; Low-level laser therapy; Low-level light therapy; Photobiomodulation; Radiometry.

Fig. 1

a Flow chart of search strategy to identify articles for review using ‘low and level and light and therapy.’ b Flow chart of search strategy to identify articles for review using ‘photobiomodulation’

Fig. 1

a Flow chart of search strategy to identify articles for review using ‘low and level and light and therapy.’ b Flow chart of search strategy to identify articles for review using ‘photobiomodulation’

Fig. 2

Examples of spatial distribution of irradiance in lasers and LED lights where the highest to lowest irradiance is represented by the rainbow colours, red to violet, respectively, for a 660 nm laser, b 810 nm laser and c 810 nm LED

Fig. 3

The definitions of radiant flux density arriving (a irradiance) or leaving (b exitance) a surface (the lines represent rays of light travelling in the direction of the arrow)

Fig. 4

Schematic representation of the internal workings of an a integrating sphere showing the 360° collection of light; b a cosine corrector probe allowing a 180° field of view; and c the internal workings of a UV–vis spectrometer

Fig. 5

A 2D beam profile image of a LLLT laser device: a an image of the actual tip area used for light delivery, b the actual active beam area and the location of the beam within the fibre optic tip and c the laser ‘speckle’ beam pattern of the devices and its active beam diameter/area