We observe colour by reflection of light from objects. In simple terms an object is blue because it absorbs all the non-blue light and reflects the blue light back to our eyes. If the light source in the area has no blue component in its spectrum then we will not detect the blue colour. A formerly common example of this was the lack of colour definition provided by low pressure sodium street lights—which is one reason why they are no longer used despite their efficacy and long life.
Work by Dain and Hood, has suggested that light sources, which will allow the reliable detection of cyanosis, should have an appropriate power output in the red part of the visible spectrum, particularly around 660 nm where the maximum difference in spectral transmittance between oxyhaemoglobin and reduced haemoglobin occurs. If the output is too low a patient’s skin colour may appear darker and he/she may be diagnosed as cyanosed when this is not the case. Conversely, if the output is too high it may mask the cyanosis and it may not be diagnosed when it is present. The end result is that clinical staff cannot rely on visual detection.
It has also been found that lamps suitable for the reliable detection of cyanosis should have a correlated colour temperature (CCT) between 3200 K and 5500 K. In general it would be expected that non-complying lamps with CCTs above 3200 K would provide false positive diagnoses and that lamps with CCTs below 3200 K would result in failure to detect cyanosis. It should be noted that, while cyanosis is defined as a bluish discoloration, 660 nm lies in the red end of the colour spectrum.
Low pressure mercury (fluorescent) lamps commonly in use may not have a continuous spectrum. Modern tri-phosphor type lamps have three phosphor coatings and have significant power output in only three regions of the visible spectrum based around wavelengths at 440 nm (blue), 540 nm (green and 610 nm (orange/light red) and may not have any significant output in the 660 nm region.
Extensive clinical trials carried out at Royal Prince Alfred Hospital in Sydney in the early 1970s identified a number of lamps that were suitable for reliable diagnosis of cyanosis. This led to the publication of AS 1765:1975 which included a graphical method of determining which lamps were suitable based on colour temperature and the colour rendering indices Ra and R13. An outline of the method can be found in AS/NZS 1680.2.5:1997 Appendix H.
The lamps identified in the 1970s used halo-phosphor technology and generally had a continuous spectrum. In the 1980s, however, tri-phosphor lamps entered the market and over a period of time have replaced halo-phosphor lamps except for special purposes. Tri-phosphor lamps provide major efficacy and life benefits.
As part of a review of AS 1680 in the 1990s, Standards Australia Committee LG/1, Interior Lighting, revisited hospital lighting. Resources were not available to carry out the large scale trials of the 1970s, which had established the original cyanosis observation criteria. However, using the data from the first trials and the known reflective properties of blood, a methodology for calculating a Cyanosis Observation Index (COI) was established and published in AS 1680.2.5:1997.
The following selection criteria should be taken into account when selecting lamps for the reliable diagnosis of cyanosis:
Lamp efficacy has a varying impact on lighting design. Many recommended lighting levels in hospitals are generally in the 160–240 lx range and these are readily achieved by lower efficacy lamps at practical spacings. For larger areas and higher recommended illuminance areas, higher efficacy can be an advantage. Lamp prices will vary, with costs to a hospital for complying lamps expected to be about 3 times the cost of a non-complying 800 Series tri-phosphor.