What is important in a surgical light?
Successful illumination requires a special balance of luminance, shadow management, volume, and temperature. This maximizes visibility at the surgical site while minimizing eye fatigue.
Prior to the invention of the electric light, the first surgical operations took place in daylight, with illumination from 10,000 lx to 80,000 lx depending on the time of day and cloud cover. By the end of the 19th century, light bulbs replaced direct sunlight and allowed more freedom to perform surgery throughout the day. By the 1960s, the use of halogen bulbs increased the amount of light available on the operating table to 100,000 lx — comparable to bright noon sunlight on a Mediterranean beach. Thirty years later, discharge lamps nearly doubled the available light to 200,000 lx! But this “more is better” approach led to eye fatigue, diminishing surgeons’ effectiveness. Today, more and more surgical lights operate with LEDs. These bulbs use less energy to produce crisper, cooler illumination that gives surgeons the visibility they need to make effective diagnostic decisions.
Proper illumination is a compromise to ensure a good visibility with minimal glare and prevent eye strain for surgeons.
Maximum central illumination (Ec) must be between 40,000 and 160,000 lx at the center of the light patch, one meter from the light source.
Light patch size can vary depending on the type of surgery to limit peripheral glare. This light patch is calculated where illumination is greater than or equal to 10% of the maximum illumination (EC). The diameter of this area is known as D10.
Illumination must be uniform to reduce glare-related eye strain. The diameter will be measured where illumination is 50% of the Ec (called D50), and a D50/D10 ratio will be given. The standard indicates that this ratio must not be under 0.5.
What is really important in term of shadow management is the usefull light the surgical team will have when operating below the lighthead and not the power of the lighthead alone.
The quality of a surgical light depends on the amount of useful light found inside the cavity. This is a balance between the amount of light, and the ability to manage shadows.
There are two types of shadows: cast shadows and contour shadows. Cast shadows interfere with visibility, while contour shadows help us to gauge depth and volume. Effective lighting minimizes cast shadows while enhancing contour.
Cast shadows are influenced by the surface of the light source, how light is guided towards the operating field, where it’s most needed, and the number of light sources. More light beams improve shadow dilution, improving visibility for the surgeon.
Volume of light
Because the surgical cavity isn’t flat, surgeons need illumination in three dimensions. Merging several light patches —shining at the same time and strength at different heights — creates a uniform volume of light.
Volume of light is measured by an IEC standard equation: L1 + L2.
- L1 is the distance between Ec and the point where luminance is measured at 60% going towards the lighthead
- L2 is the distance between Ec and the point where luminance is measured at 60% going away from the lighthead
The best surgical light will have the highest volume of light at the cavity, even when the light is positioned more than one meter from the surgical site.
L2 measurement is particularly critical for deep cavity applications.
Light will be always energy so in order to prevent from tissue desiccation, a safe heat management is mandatory.
Heat can be measured in two places: at the light patch, and at the lighthead. Heat can cause discomfort for the surgeon, and can dry exposed patient tissue.
While LEDs do not produce damaging infrared rays, some heat remains. The more illumination, the more radiant energy. The IEC limits this irradiance to no more than 1000W/m2 at the light patch. Overlapping light patchs is always under surgeons’ control.
But lightheads are also a source of radiant heat, which needs to be reduced to prevent overheating and laminar flow disturbances.
A quality surgical light will minimize heat to improve comfort and patient outcomes.
Fail safe surgical lighting
There is no room for error in the surgical suite. Fail-safe equipment is designed to ensure that even in a single fault condition no safety hazard exists. It ensures that illumination and maneuverability are preserved, providing central illuminance of not less than 40,000 lx.
Single luminaries without any protection against light interruption under a single fault condition are not fail safe. A light with two minor luminaries with separate transformers, fuses, wiring, and slip rings would be fail safe.
Examples of single fault condition:
- Breakage of wire inside the equipment
- Failure of slip ring, or a fuse, or a lamp, or insulation
- Failure in electronic device
- Lamp power supply cables detachment