Having looked at the rules for force fields (in Vehicles), the rules for
space ship force fields (in Space) and the rules for personal force fields
(UT/Space) I'm impressed by how...incompatible they are.

Ultratech screens are fairly non-linear in DR; a backpack force screen has
the comparable weight and cost to a 20 sf vehicles screen, but only draws 35%
of the power; however, a personal screen weighs 1/5 as much as a comparable
vehicles screen, and costs 60% as much; a heavy screen generates a field over
a 5 yard radius (2800 sf), and as a vehicles screen would weigh 2.8 _tons_.
Space screens are quite heavy (assuming that 1 DF space is ~100 DR), but don't
increase in weight very fast as they protect a larger area; they appear to
be roughly order 1/3 in volume (rather than 2/3).  This encourages putting
screens on large vehicles (Vehicles screens are _much_ too expensive for that).

Thus, I've come up with variant rules for buying force fields; also, variant
rules for screens which don't behave like the standard screens.

When designing a force screen, decide on the following properties:
Tech level: tech level screen was designed at.
DR: the DR of the intact screen.  This cannot be less than 100.
Radius: the maximum radius (in yards) of the screen.  The minimum radius is
    four yards or DR/50 at TL11, halving at every subsequent tech level. 
    Personal screens have a radius of at least one yard.  All screens may be
    used at less than their maximum radius if desired; this is frequently
    true of personal screens.
Burnout behavior: when a force screen is overloaded, does it:
    -explode, doing 1d per kW of power required.  Normally, if this kind of
    screen is used, it is the only type of screen available.
    -burn out, requiring total replacement.  The screens listed in UT are this
    type.
    -overload, taking its HT in damage; it is considered disabled, but may be
    repaired normally.  An overloaded screen will radiate away its absorbed
    energy in 18-TL minutes; working on it before then is quite dangerous.
    This is the standard TL11 space ship screen.
    -blow a fuse, taking 1d% of its HT in damage; it is considered disabled
    until replacements are made.  An overloaded screen will radiate away its
    absorbed energy in 18-TL minutes; fuses cannot be replaced before then.
    Not available until TL 12, at which point it is the standard space ship
    screen.  Replacement fuses are 5% of the cost, weight, and volume of the
    screen.
    -shut down.  Requires 18-TL minutes to cool down, at which point it may be
    reactivated normally.  Not available until TL 13, at which point it is
    the standard space screen.
    -explosive dissipation: identical to one of the above three, but energy
    stored in the screen explodes outwards, this may cause damage to people
    outside the screen.  Base damage is identical to exploding screens, above,
    but is divided by the square root of distance as per a nuke flash.
Compact: reduces weight substantially, with a substantial cost penalty.
    It is assumed that a compact screen does not include normal cooling
    systems; these screens are _always_ destroyed if they are overloaded,
    and have a quite small maximum size; in addition, due to the lack of
    cooling they require access (really cooling) space equal to their volume,
    as per drives and power plants.  In addition, compact screens have a
    fairly short MTBF, and should be shut down at least once an hour.

Weight: the weight of a screen is equal to D * R * T * C, where:
D is equal to DR.
R is equal to radius.
T is 2 at TL11, 1 at TL12, 0.4 at TL 13, 0.2 at TL14, 0.1 at TL 15+.
C is 0.05 if compact, 0.25 for burnout, otherwise 1.0.  Double weight
of burnout screens at TL 11.

Power: the power requirement of a screen is equal to D * R * 0.5.  A screen
    in standby mode (i.e. not currently under attack or discharging) draws
    only 20% of its regular power requirement -- this will frequently be
    provided by power cells, though this risks loss of power and system
    damage if the screen remains under attack for too long.
Volume: equal to (weight/100).  Compact screens require access space equal
    to their volume, unless located in a pod.
Cost: weight*20 for standard screens, weight*50 for burnout/exploding screens,
    weight *1000 for compact screens.  Halve price for overload screens at
    TL 12+ or fuse screens at TL 13+.

Samples:
TL11 PF 1 force field, 10 cy ship: 5 yards * DR 200, overload.  2000 lb,
    $40,000, draws 500 kW, 100 kW on standby.  This is half the weight of
    a TL11 screen, with dramatically lower power consumption; the cost is
    identical.
TL13 PF 1 force field, 10 cy ship: 5 yards * DR 200, shutdown.  400 lb,
    $8,000, draws 500 kW.
TL13 backpack force screen: 2.5 yards * DR 500, burnout, compact.  25 lb,
    $25k, draws 625 kW; a D cell lasts 15 minutes standby or 3 minutes combat.
TL13 personal force screen: 1 yard * DR 200, burnout, compact.  4 lb,
    $4k, draws 20 kW; a C cell lasts 10 minutes standby or 2 minutes combat.
TL13 heavy force screen: 5 yards * DR 1000, burnout, compact.  100 lb, $100k,
    draws 500 kW; an E cell lasts 40 minutes standby or 8 combat.

Special rules: explosions.
    When dealing with an explosion, you should _square_ the screen DR.  When
    dealing with the flash damage from a nuclear weapon, add half the radius
    of the screen to the computed damage.
Special rule: explosions.  When dealing with concussion damage, square DR
    _before_ computing the number of energy levels gained; you should also
    square DR when computing damage from nuke flash damage.
Special rule: turning off a screen.  Any screen (except, generally, an
    exploding screen) may be may be turned off.  Exploding screens will
    explode; multiply damage by percent full the screen was.  Burnout screens
    will burn out; overload screens may take a percentage of damage equal to
    the percentage of the max energy states the screen was at, or may simply
    behave like one of the below types.  Fuse and shutdown screens will simply
    turn off; divide the normal rate of energy loss by (18-TL); the base power
    requirement of the screen must continue to be paid until the screen
    discharges fully.  Any screen which loses power will turn off, and will
    _automatically_ suffer damage, per burnout or overload screens.
Special rule: adjusting screen power.  Screens can generally be run at
    less than maximum power; if you switch strength while under attack,
    shift the energy levels proportionally (this can cause a screen to
    overload).

Variant force fields:
    The force field rules in GURPS simulate one particular type of force field
(typical of Lensman); in my experience it isn't even the most common.
Here are a few variants:
1) Variant Penetration/Overload behavior:
The standard screen has 8 energy levels (8 being burnt out), and gains an
energy level for every half its DR.  Alternates:
a)  No burnout -- screens are not subject to overload.  Double weight and
    power requirements (or, for a predesigned screen, merely halve DR).
b)  10/100: screens go up 1 energy level when they take 10% of their DR in
    damage, and lose energy levels at 1 per second; they have 100 energy levels.
    DR of a screen can never exceed the amount which would overload it.
    Mass and power requirements are normal.
c)  100/100: screens go up 1 energy level when they take 1% of their DR in
    damage, and lose energy levels at 1 per second.  DR is limited to overload
    DR, as above.  Divide weight, power requirements, and minimum radius
    by 5 (alternately, take existing screen and multiply DR by 5).
d)  100/100I: as above, but the screen can typically _only_ be penetrated by
    overloading it.  DR is not affected by divisors (optionally, armor divisors
    are reduced by a factor of 10).  Weight is reduced as above.

2) variant shoot-through behavior:
    The 'standard' screen does not block any energy projection; it also allows
through enough energy for sensors (for some obscure reason, passive sensors
looking outwards are disabled, but active sensors aren't...given that an
active sensor is simply a passive sensor + a flashlight, its not obvious
why this would be true).  There are a variety of possible variants for
this:
1)  the 'threshold' screen: the screen allows through energy up to a (fairly
    low) threshold energy, and becomes opaque at higher energies; it might
    become opaque in all bands, or only in a few.  This is not actually a
    variant, I assume it is true for the standard screen.  This is actually
    true for most screens.  A screen which is not a threshold screen will
    blind the person inside (it is 'opaque').
2)  the 'dual' screen: this screen blocks attacks in both directions, and
    must be lowered before you can attack; this will also block use of reaction
    drives, and _may_ prevent reactionless drives from taking effect.  It 
    generally has no effect on contragravity.  Most dual screens use some
    variant which can be raised or lowered.  A dual screen which is not a
    threshold screen is opaque in both directions.
3)  tuneable screens: this is a dual screen which can be tuned to be transparent
    to a particular frequency of energy; this allows firing out with any rainbow
    laser or xaser which is linked to the screen computer; in the case of an
    opaque screen it also allows vision in that band.  It also allows firing
    _in_ if you can determine the wavelength the screen is tuned to (simple
    spectral analysis will probably reveal this, so tuneable screens generally
    shift frequency rapidly).  Tuneable screens may have computerized
    frequency shifting, which can be solved for by a fast computer (frequency
    shift: C=1+, cost = 5k; frequency analysis: C=2+, cost = 20k); the chance
    of shooting through a shield is (6 + 2*C(analysis) - 2*C(shift)), so 6-
    for equal complexity.  If you do not have a frequency shift program treat
    C as 0, and after 1 success no more rolls are required.  Frequency
    analysis is not possible through opaque tuneable screens.  This is a
    fairly uncommon type of screen.  Note, also, that some kinds of screens
    which allow firing normally _also_ permit frequency analysis; this will
    either permit firing through the screen, or will have the effect of
    setting up resonances in the screen (double weapon damage for purposes
    of screen overloading).
4)  fast flicker screens: this is a dual screen which can be turned on and off;
    it may or may not be opaque.  It is possible to fire energy weapons which
    are slaved to the screens, as above; flickering can also be analysed just
    like frequency shifting.  In addition, simple luck allows shooting through
    flicker screens; treat it as an active defense, with a defense roll of
    (complexity + 10).  Flicker screens can automatically be turned off at
    no penalty, and cannot be burnout screens. A flicker screen can also be
    left off, and used as a block against projectiles; this requires
    the equivalent of a gunner program, which rolls against its skill; this
    mode is useless against beams.  Traveller black globe generators are
    opaque fast flicker screens with absorbtion (see below).
5)  slow flicker screens: generally opaque, and very similar to the above;
    however, it can't be flickered many times a second; this makes it a game
    of guessing when screens will open.  This type of screen isn't very
    interesting if energy weapons are predominant, as you can just sit
    there with guns locked waiting for the screen to open.
6)  portal screens: this is a dual screen which you can deliberately open
    holes in to fire out of; it is usually not opaque.  Portals typically
    don't close all that fast, though it takes less than a second; thus,
    give the gunner an active defense roll (based on skill/2) to close the
    hole before it is possible to shoot through.

Variant defensive behavior:
a)  limited coverage: screens will sometimes not stop various substances;
    the most traditional substance is 'slow-moving objects'.  Other options
    I have seen are 'only stops energy', 'only stops physical objects',
    'stops everything'.
b)  absorbing screen: an absorbing screen _generates_ power when it is
    in the process of discharging; power production is equal to (minimum
    damage to go up one energy state)^2/(time to go down 1 energy state)/
    50,000.  If you cannot use this power, the screen does not discharge.
    This is a particular problem with dual screens, as an absorbing dual
    screen also traps heat within the system, so any power-using system
    can just be assumed to either feed back into the screen or to generate
    heat -- you cannot dispose of power by firing energy weapons.

Some of the most common screen types:
Star Trek: screens should be 100/100I, threshold screen.
Traveller Black Globe: 100/100I, dual, fast flicker, absorbing, explodes.
Traveller White Globe: 100/100I, threshold, burnout, one-way.
Star Wars: 100/100I, choice of 'energy only' and 'physical only'.  Physical
screens are dual screens, energy screens are threshold screens.