We have measured the brightness (luminance) of the light spot produced by 105 Macintosh 3 laryngoscope blades (33 bulb, 72 fibrelight) using a Hagner photometer. An estimate of the minimum luminance required for laryngoscopy (circa. 100 cd.m-l), was determined using a laryngoscope adapted to provide a variable light output. Five (15%) of bulb blades and 24 (33%) of the fibrelight blades failed to meet this minimum level. A new bulb blade produced brighter lighting conditions (maximum 700cd.m-') than a new jibrelight blade (max. 500cd.m-'). In total, 61 (84%) of the fibrelight blades and three (9%) of the bulb blades were ,found to provide a light spot that encompassed areas of luminance below 3Ocd.m-', which is a level for comfortable reading, The light spot from a mains-powered fibreoptic bronchoscope was found to be four times brighter (2000 cd.m-') than a new battery-powered jibrelight laryngoscope.
SummaryWe have measured the brightness (luminance) of the light spot produced by 105 Macintosh 3 laryngoscope blades (33 bulb, 72 fibrelight) using a Hagner photometer. An estimate of the minimum luminance required for laryngoscopy (circa. 100 cd.m-l), was determined using a laryngoscope adapted to provide a variable light output. Five (15%) of bulb blades and 24 (33%) of the fibrelight blades failed to meet this minimum level. A new bulb blade produced brighter lighting conditions (maximum 700cd.m-') than a new jibrelight blade (max. 500cd.m-'). In total, 61 (84%) of the fibrelight blades and three (9%) of the bulb blades were ,found to provide a light spot that encompassed areas of luminance below 3Ocd.m-', which is a level for comfortable reading, The light spot from a mains-powered fibreoptic bronchoscope was found to be four times brighter (2000 cd.m-') than a new battery-powered jibrelight laryngoscope. Key words Equipment; laryngoscopeThe Macintosh laryngoscope blade was first described in 1943 [I] and is used by most anaesthetists for laryngoscopy during tracheal intubation. A recent theoretical analysis of blade design scored Macintosh blades highly [2]. In addition to the shape of the blade, visualisation of the larynx at laryngoscopy is dependent on the amount of light emitted from the blade. We were concerned that some blades in use provided very little light and have measured the luminance (surface brightness) of the light spot produced by 105 blades in everyday use in our hospital.The SI unit of luminous intensity is the candela. A number of other terms and derived units are used to quantify light; further details of these are given in the Appendix. Luminance is the term used to describe the amount of light reflected from a surface which determines our perception of surface brightness. It is measured in SI units of candela per square metre. MethodLuminance measurements were made using a Hagner universal photometer model S 1 (Hagner International U.K. Ltd Ringwood, Hampshire). It is shown in Figure 1. This instrument has a sensitivity that approximates to that of the human eye. It functions by filtering light received through a I" circular viewing angle onto a photocell via a series of filtered lenses. The resulting electrical current is then measured on a sensitive galvanometer scale. The scale is calibrated to allow luminance values to be read directly from a meter on the side of the instrument.Measurements were conducted under photographic darkroom conditions using the arrangement illustrated in Figure 2. The test configuration was designed to model the clinical setting of laryngoscopy. A target glottis was drawn on a white gloss surfaced card to a size of 40 mm circular diameter, calculated to be the approximate size of the target glottis in the clinical setting (Fig. 3). Light spot measurementsLuminance measurements were recorded at two points; at the centre of the light spot [C], and at a point 20 mm from the centre, at the periphery of the target glottis [PI.Each laryngoscope blade was teste...
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