Optics

Good optical design is important in a flashlight. It guarantees that as much light as possible from the light source is collected and directed into the beam of light. The more efficient the design, the brighter the beam appears, and the less battery energy consumed. There is no single perfect flashlight. Many variations are possible depending on what you are trying to see with the aid of the flashlight.

Beam Appearance The spot of light cast by the flashlight can appear in different ways. It can be large or small, evenly distributed or brighter in the center as well as smooth or patterned. Different applications require different beams. Spotting distant objects works best with a narrow, intense, center weighted beam. Map reading requires a low intensity, uniform, wide beam. Lights which use reflectors with mirror like finishes tend to produce beams with bright hot spots. Lights with reflectors which appear textured or dimpled are less bright than lights with mirror finish reflectors, but the beam is much more smooth and uniform.

Spill Light Most flashlight beams consist of two zones. The center area is a bright circle which gets its brightness from all the light collected by the reflector and sent forward into the beam. There is also a much larger and less bright circle of light called the spill light which comes from light released by the lamp that travels directly outward without being reflected into the beam. The amount of spill light can be useful or detrimental. Spill light is useful at night for illuminating the area around the central beam for good peripheral vision. Underwater or in foggy and smoky environments the spill light illuminates the particles surrounding the central beam and makes it more difficult to see.

Color Temperature

The color temperature of a light simply describes the color of the light beam on a scale which starts yellowish (low color temperature) and goes to bluish-white (high color temperature.) It is expressed in degrees Kelvin (ï¿½K.) Direct sunlight can be 10,000 ï¿½K whereas light from a candle is close to 1,500 ï¿½K. Incandescent lamps cannot exceed 3,400ï¿½K or they will burn out. Most high performance xenon and xenon halogen lights hover around 3,000-3,200 ï¿½K. Inexpensive or low performance lights can be as low as 2,000 ï¿½K. White LEDs and HID lights are in the range of 5,000 - 7,500 ï¿½K.

As batteries drain the color temperature of an incandescent lamp decreases whereas LED and HID lights remain nearly constant. This can be useful for photographers who need constant color temperature.

Brightness

There is no single number or measurement which can specify the brightness of a flashlight. As a minimum, it is necessary to specify the total amount of light released, the beam angle and the brightness of the center of the beam. It is important to understand the difference between the terms used to describe brightness when making a selection.

Wattage is a measurement of the power delivered by the battery. It cannot be directly converted to brightness because different lamps have different efficiencies. Some give off more light than others per watt of power. Also the wattage changes as the battery is expended. As the battery loses its charge the wattage decreases to zero. Typically flashlight wattage is expressed for fresh batteries at peak charge.
Lumens (lm) is a measurement of the total light given off by the lamp. It is not, however, a measurement of the intensity of the beam. It includes the sum of all the light in the center beam and the spill light. Higher numbers mean greater overall illumination.
Beam Angle (ï¿½) is a measure of the spread of the central beam not including the spill light.
Beam Candlepower (CP, cd, CBCP) is the measure of the actual brightness of the center of a beam. There is always a tradeoff between the brightness of the beam and the beam angle. Two lights with the same lumens and wattage values can have very different beam candlepower measurements due to the fact that one has a large beam angle and the other has a small beam angle. This can be explained by knowing that the light is spread over a wider area in the large beam angle light and is not as bright as the small beam angle light. Flashlights with extremely high CP (beam candle power) ratings typically have very small beam angles.
Luminous Efficiency (lm/watt) measures how efficiently light is produced from electrical energy. In simplest terms one divides the lumens output by the wattage input. Different sources of light have different efficiencies. Incandescent lamps used in flashlights often range from 10-25 lm/watt. HID can be 40-50 lm/watt. LED efficiency increases by the month of manufacture. Older white LEDs produced 6 lm/watt. Insiders predict LEDs will go as high as 200 lm/watt in the next few years. In 2006 they cluster around 20-40 lm/watt.

UK Optical Designs

At UK we use a variety of designs to optimize the performance of a light for its intended task. Here are some examples.

Parabolic Reflector

A reflector in the shape of a parabola is often used in UK flashlights like the 4AA, 2AAA, UK300 and SL4. If the lamp is placed at the exact focal point of the parabola all of the light which comes from the lamp and bounces off of the reflector is directed straight forward into a beam which follows the axis of the reflector. The beam spreads slightly because the filament of the lamp is larger than the focal point. Light from the portion of the filament not on the focal point deviates in its path making the beam wider as it gets farther from the reflector. A narrow bright beam is the result.

Lamp Shuttle

In order to provide a back-up lamp or selectable high and low wattage lamps a mechanical shuttle was designed for lights like the C4, C8, D4 and D8. It moves the selected lamp into the focal point of the reflector and turns it on. The other lamp remains to the side and out of the way in the off position. By rotating the switch the resting lamp is shifted to the focal point and the other lamp moves to the side.

Reflector Surface Texture

The reflector surface of our larger lights such as the C8 or D8 is not perfectly smooth. Small dimples are applied which cause light to be scattered slightly when it is reflected into the beam from the lamp. By carefully controlling the size and shape of the dimples the distribution of light can be controlled in a way to affect beam angle and smoothness of the beam.

The reflector used in LED lights such as the C4 or C8 eLED uses a slightly different strategy to control beam spread and smoothness. Slight deviations from a parabolic surface are applied in concentric rings around the reflector. This approach works well with the shape of an LED which is quite different from a filament lamp.

Bezel Curvature Underwater light is bent as it travels out from the front port of a flashlight. If the port is flat, light bends toward the axis of the beam and the light beam becomes more narrow underwater than it is out of water. By applying a slight outward curvature to the port, light can be bent away from the axis, and the beam angle remains the same in or out of water. Bezels for the C4, C8, D4, D8 and Light Cannon have curved ports.

Zoom Optics

The challenge was to create an efficient optical system which allowed the user change the beam angle of a LED flashlight. Moving a lamp in and out of the focal point of a parabola to change the beam angle of a flashlight is common practice. The only problem is that as the beam becomes wider a dark spot appears in the center of the beam. In order to maintain an evenly illuminated beam we developed a set of lenses for the purpose. The first lens is placed almost in direct contact with the LED. It captures almost all of the light from the LED and directs it forward into the beam. A second lens was designed to be placed in front of the first to further reduce the beam angle. By changing the distance between the first and second lens the beam angle can be adjusted between very narrow and wide. This system is used the the 4AA eLED ZOOM lights. The 4AA Mini Q40 eLED PLUS and 4AA eLED Fire lights utilize similar optics, but the lenses are permanently fixed in the narrow beam position. This type of optics also eliminates any spill light because the lenses collect all of the light and direct it into the beam.

C8 eLED Plus
combination reflector

Multiple LEDs

Unlike incandescent lamps, single LEDs are limited in brightness. The only way to achieve a high intensity LED flashlight is to combine the light from several LEDs into a single beam. Early designs included large arrays of small LEDs arranged on a flat plate and aimed in the direction of the beam. This resulted in a diffuse wide beam. The C8 eLED PLUS incorporates a combination reflector with a high intensity LED positioned at each of two focal points. Light from the LEDs is reflected and directed forward into a relative narrow intense beam with much greater efficiency than the older arrayed design.

OPTICS