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Luminous Intensity
One of the major differences between HI and regular LEDs is in luminous intensity. This is the measurable power emitted by a light. The units of luminous intensity are millicandelas (mcd). A regular red LED has a typical luminous intensity of 65mcd, whereas red HI LEDs can have a luminous intensity of 100mcd. The brightness of regular LEDs are appropriate for regular tasks such as power indicator lights, but in cases in which the clear visibility of the light is important over a distance, HI LEDs work better.
Color
Wavelength determines the color of a light. Both HI and standard LEDs come in one of seven colors: red, orange, amber, yellow, blue, green or white. A standard red LED has a wavelength of around 655, whereas HI LEDs have wavelengths in the low 640s.
Forward Voltage
Forward voltage is the amount of electrical power that runs through the LED when power is applied to the circuit. Regular LEDs have a forward current of about 1.8 and can take a maximum forward current of 2.2 before burning out. HI LEDs have about the same threshold in terms of typical and maximum forward current. This is a useful property because when replacing a standard LED with an HI LED or vice versa, one usually won̵7;t have to change the resistor that limits voltage to the LED bulb.
Viewing Angle
The viewing angle of an LED is the angle at which an observer will see the light at at least half the level of brightness compared to what he would see if he were standing directly in front of the light. Most standard red LEDs have a viewing angle of 60 degrees. This means that the observer could move 30 degrees either to the right or left before the level of brightness would drop by more than half. For HI LEDs, the viewing angle ranges from 20 to 35 degrees. This is an important consideration for any circuit designer because while they would gain brightness using an HI LED, there is a trade-off in visibility at alternate angles.