July, 2018: Light Intensity

Light is a fascinating thing.  From the dual nature of light as a particle and wave, to the combination of colored light to make new colors, it is an illuminating element that we often take for granted.  How often does that average person consider the color-temperature of their office lamp for instance?  In Shedding Light, we aim to bring understanding to issues surrounding illumination.  In March’s issue we discussed coaxial lighting and Pure Light Technology (Color Temperature), and in this issue we will be discussing light intensity.
The first thing to know about light intensity is the language that surrounds the topic.  Many have probably heard the term lumens (and some may have heard foot-candle), but in the modern world of surgical lighting “lux” is king.  Lux is the International System of Units (SI) measure of illuminance and refers to the number of lumens per square meter.  The area part of the equation is crucial in defining intensity.  If you produce the same number of lumens (think amount of light) over a larger area, it will appear dimmer and less intense.
Fig. 1
Intensity and area of illumination lead to one of the most crucial factors in rating a surgical light source: working distance.  To start this discussion, we will use the analogy of a car headlight.
A car’s headlight emits the same amount of light all the time, but the farther away from the source you go, the more area the light inhabits, and the lower the intensity. Think about the driver of a car:  the road directly in front of the car is very well illuminated, but farther down the road everything is still dark.
Fig. 2 (2)
When it comes to surgical lighting, the same is true. An LED Headlight will appear much more intense at a close working distance than at a far one.  If you were to put on a Surgical LED Headlight and shine it at a white wall your eyes would not feel very good two inches from the wall; It would get easier on the eyes as you moved away.
To exemplify this fact, we looked at Enova’s brightest LED Surgical Headlight: the XLT-225A.  The 225 in this headlight’s name stands for 225,000 and refers to the lux output at a working distance of 14” (35.6 cm).  This headlight produces 225,000 lux 14” from the source, an average working distance of many surgeons.
To show how intensity decreases over distance from the source, we used a lux-meter to measure output at distances other than 14”.  Below you will find the data showing the same effect as the car’s headlights. LED headlights appear much brighter the closer you are to the source.
Fig. 3
What does this mean for surgeons and other headlight users?  It means the intensity of light that you experience is directly related to the distance at which you work, not just the output of the headlight you use.
When trialing or purchasing new headlights, it is important that you consider the reported working distance of the headlight, as well as your standard working distance as a user.
We consider 14″ (36 cm) to be a standard working distance for measurement, but the industry has no defined standard, and some headlights are reported at 12″ or less. An LED headlight may say (in the case of our XLT-125A) that it puts out 125,000 lux at 14”, but if you work at 12” it will appear much brighter than that.
The aim of our articles is to increase your knowledge and understanding of LED Surgical Lighting and the communities that use it.  After reading July’s issue we hope you better understand the numbers associated with these devices.  At the very least we hope that you have been intrigued by the phenomenon of illumination.  It still intrigues us here at Enova.
emma enova

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