How to Build a Laser Death Ray
Anti-personnel death rays
If you want to directly use your death ray to mow down the minions of your enemies, you have a couple of choices of how to design your über-weapon.
The first method involves building a heat ray and using it as something like a long range flamethrower. Heat up the victim's skin until it burns, or his clothes until they ignite. The gruesome details have been investigated during the Cold War to determine the effects of nuclear blasts, one of whose primary means of killing is by the radiant heat emitted by the fireball. The energy needed to cause ignition varies with the composition and color of the target and the radiant intensity and duration of the thermal pulse (longer pulses allow more time for the heat to diffuse into the target, thus lowering the surface temperature - on the other hand, a very shallow heated layer may not sustain combustion). The NATO HANDBOOK ON THE MEDICAL ASPECTS OF NBC DEFENSIVE OPERATIONS AMedP-6(B) PART I - NUCLEAR lists the radiant flux for various yields of nuclear explosives needed to ignite various fabrics, and notes that "where the radiant thermal exposure exceeds 125 Joules/sq cm, almost all ignitable materials will flame."
Details on the radiant intensity needed to cause burns to skin can be found in the Nuclear Weapons Frequently Asked Questions website. For flashes lasting between a fraction of a second and several seconds, between 35 J/cm2 and 50 J/cm2 is the threshold for third degree burns (burns which destroy the full thickness of the skin) while between 20 J/cm2 and 35 J/cm2 is the threshold for second degree burns (burns which destroy a partial thickness of the skin). The threshold for first degree burns is given as between 10 J/cm2 and 20 J/cm2. The same web page notes that at irradiances of roughly 400 J/cm2, the thermal radiation is sufficiently intense to "cause exposed flesh to flash into steam, flaying exposed body areas to the bone."
First degree burns cause pain, but are not usually serious. Second or third degree burns covering more than 15% of the body are very serious and will likely result in death if not given medical attention. Incapacitation will be rapid and shock can be expected within minutes. The percentage of skin burned can be estimated with the rule of nines, which assigns 9% of an adult's skin surface area to the head, 9% to each arm, 9% to each upper and lower leg (so 18% for each entire leg), 36% (four 9% sections) to the torso, and the remaining 1% to the genitalia. Since it is likely that the victim of a laser death ray will only be burned on the side facing the death ray generator, the percentages given should be halved.
A 60 cm spot will cover the torso of most adult humans, allowing about 20% of the body to be affected in one exposure. This corresponds to a spot area of about 3000 cm2. An intensity of 125 J/cm2 would be quite effective - uncovered skin will suffer devastating flash burns while exposed hair and clothing will burst into flame. This will take about 375 kJ of energy per kill. An intelligent targeting system that could irradiate only exposed skin could get by with somewhat less energy expenditure.
The second method involves blasting through skin, meat, gristle, and bone to make holes in vital organs. This is much the same mechanism as bullets use. Experts recommend that when chosing a bullet and firearm for home self defense (or for police), the bullet should be able to penetrate 30 cm of soft tissue. Anything less is unreliable or even ineffective at going deep enough to damage the heart, central nervous system, or major blood vessels. It is generally recommended not to rely on pain, psychological shock, or massive but shallow injuries to incapacitate your target - tough guys will keep on coming unless they are physiologically unable to keep moving and this means disabling the central nervous system or causing hypovolemic shock (basically, severe blood loss).
Much like with modern firearms, shots which hit the target but miss vital organs may not cause rapid incapacitation and may not even kill over the long term. Shot placement is vital. If you are not using robot weapons directed by automated targeting systems, you will want to train your own minions in accuracy and shot placement under stress and in exceptional conditions so that they can perfrom while terrified, under fire, crawling through mud, and when they can barely see their opponents.
Assuming a hand-held blaster designed to take on the role of a modern assault rifle, you might choose a design with a 6 cm primary focusing aperture which emits a train of one hundred pulses spaced 1 μs apart, each with an energy of 10 J and a duration of 1 ns. The damage calculator indicates that if focused to a spot size of 10 mm diameter, you can expect it to blast a hole of about 6 cm across and about 30 cm deep through meat - sufficient to meet the 30 cm depth criteria. With a 6 cm aperture, it could emit laser light at an eye-safe 1.5 μm wavelength and still focus to a 10 mm spot size at a distance of over 300 meters (as a mad scientist or evil overlord, you don't care about the eye safety of your enemies. However, it is a good idea to worry about the vision of your own troops, and 1.5 μm allows you to have your minions use their weapons even if they forget their protecive eye-wear). Most infantry engagements take place at less than 300 meters or less, so these design parameters fullfill the criteria for an infantry rifle-equivalent. A 10 mm spot diameter performs rather poorly against armor, but at ranges of 100 meters or less the spot size goes down to below 3 mm at which point the penetration exceeds that of modern infantry rifles using ball ammunition (while increasing the penetration in meat to about 60 cm). At 50 m range, a spot size of 1.5 mm gives over 3 cm of penetration through armor steel, exceeding that of the best modern tungsten carbide armor piercing bullets fired from infantry rifles. If your troops brought their protective goggles, they could switch their lasers to 0.5 μm wavelength blue-green beams and triple the ranges - although this will give away their positions to the enemy for return fire.
The above considerations show that you get roughly equivalent performance to a modern infantry rifle with about 1 kJ of energy absorbed by the target. Factoring in inefficiencies in the laser generator and coupling of the beam to the target, you might be looking at a few kJ of input electrical energy. Compare this to the 1.8 kJ of kinetic energy of the bullet from a 5.56×45mm NATO cartridge used by the M-16 assault rifle, generated by the deflagration of about 6 kJ of smokeless powder.
It also shows that while an anti-personnel laser can mow down unarmored cannon-fodder at long ranges, for armored foes you need to get closer - possibly a lot closer. Your penetration of armor is exceptional at ranges of tens of meters, but this involves getting up close and personal with the enemy.
A final method of incapacitating the enemy is to not worry about injuring him, but just shine your lasers in his eyes to blind him. More details are given in the page on eye damage, but the basic conclusion is that rastering a beam across the battlefield could blind anyone looking in its direction who isn't wearing protective gear. This is forbidden by current international treaties - Protocol IV of the 1980 Convention on Certain Conventional Weapons specifically forbids lasers designed to blind enemy soldiers although accidental blinding by lasers intended for other uses (such as burning or blasting enemy soldiers) is perfectly acceptable.