Compared to DC operation, it is sometimes possible to significantly increase visible LED brightness by pulsing. There are 3 reasons why pulsing can increase brightness: 1) LEDs have more output at lower temperatures. Low duty factor pulsing can, in some cases, lower the operating temperature of the LED. 2) The human visual system is non-linear. With the right choice of duty factor and pulse rate, perception will correspond more to the peak brightness than to the average brightness. This is especially true at low pulse repetition rates. However, there are a couple of potential problems with using low rep rates: a) Visible LEDs may appear to flicker, especially at frequencies below 30 Hz. B) Flickering sources may be a hazard to epileptics. For more information, see the following sources: : British Epilepsy Association Photosensitivity Info 3) Many LEDs have non-constant luminous efficiency. Luminous efficiency is the ratio of light output per milliamp of input. At very low and very high current levels, you get less light per milliamp than you do at intermediate currents. From the above considerations, we can make the following generalizations: 1) If you are trying to operate visible LEDs at maximum possible brightness levels, you will always get better results using DC operation. This is because luminous efficiency declines at high currents. 2) If you are trying to achieve a compromise between battery life and LED output, i.e., you are not operating the LED at maximum power, pulsing is probably advantageous. If the LED datasheet does not provide an efficiency curve, you will have to experiment to find the best compromise.