Page 57 - Sensors and Control Systems in Manufacturing

The Role of Sensors in the 21st Century
1.10.4 Kelvin Color Temperature 19
The Kelvin color temperature is a measure of the color of a light
source relative to a blackbody at a particular temperature expressed
in degrees Kelvin (K). Incandescent lights have a low color tem-
perature (approximately 2800K) and have a red-yellowish tone;
daylight has a high color temperature (approximately 6000K) and
appears bluish (the most popular fluorescent light, Cool White, is
rated at 4100K). Today, the phosphors used in fluorescent lamps
can be blended to provide any desired color temperature in the
range from 2800K to 6000K. Lamps with color temperatures below
5000K tend to be more yellow/red, lamps rated between 5000 and
6000K are viewed as white, while lamps above 6000K tend to have
a blue cast.

1.11 LEDs—Light-Emitting Diodes
An LED is a solid-state device and does not require the heating of a fila-
ment to create light. Rather, electricity is passed through a chemical
compound that is excited and thus generates light. LEDs are not bulbs
or lamps. LEDs require precise manufacturing operation to make them
ready to be used. They need to be placed on a circuit board or other
material that will allow electricity to pass through it at a specific voltage
and current, and with components required to operate them at specific
voltages such as 12 V DC, 24 V DC, or 120 V AC. They are not readily
available to be plugged into a 12-volt or 120-volt power source.

1.11.1 LED Bars
An LED bar is a solid strip of material to which LEDs have been sol-
dered, along with resistors and other components that a specific
product requires to make them operate at the stated operating volt-
age. The bars are usually an enclosed strip of LEDs. Enclosures are
plastics, or aluminum, or metal composites with various types of
lens/cover plates.

1.11.2 LED Clusters or Arrays
A group of LEDs set in a square, rectangular, or linear pattern, and
formatted to be operated at a specific voltage. They will always
include two wires called leads. One is positive, the other negative.

1.11.3 LED Drivers
LED drivers are current control devices that replace the need for resis-
tors. LED drivers respond to the changing input voltage while main-
taining a constant amount of current (output power) to the LED as its
electrical properties change with temperature.