Light Levels

Part of Lighting

Measuring and matching illumination to task requirements to use power efficiently.

Why This Matters

Power is scarce in a rebuilding community. Every watt spent on over-lighting a storage room is a watt unavailable for the workshop. Conversely, insufficient light causes eye strain, errors, and accidents at critical work. The science of illumination gives you precise tools to match light output to task requirements — specifying how many lumens you need, where to place them, and how to verify you have reached your target.

Understanding light levels also helps you compare light sources across wildly different technologies. A single number — lumens — lets you compare an oil lamp to a car headlight to an LED strip. The lux measurement at a work surface tells you whether that number of lumens is actually landing where you need it.

In a resource-constrained system, you will likely run insufficient light rather than sufficient. Knowing the minimum acceptable levels for each task type lets you prioritize and argue for the electrical capacity needed to do work safely and well.

Units of Illumination

Lumen (lm): Total light output from a source, measured in all directions. A standard candle produces about 12 lumens. A 10W LED bulb produces 800–1,000 lumens. This number tells you how much light is available but not how concentrated it is.

Lux (lx): Illuminance — lumens per square meter falling on a surface. One lux = one lumen spread evenly over one square meter. This is what matters for task performance. The same 800-lumen bulb placed 0.5 m above a desk produces much higher lux at the desk than if placed 3 m away.

Foot-candle (fc): The Imperial unit of illuminance. One foot-candle = 10.76 lux. Common in older American engineering documents. If a reference says “30 foot-candles for office work,” that is approximately 320 lux.

The inverse square law: Illuminance falls off with the square of distance from the source. Double the distance from a point source, and illuminance drops to one-quarter.

E = I / d²

Where:
  E = illuminance in lux
  I = luminous intensity in candela
  d = distance in meters

This law means that lowering a lamp over a work surface dramatically increases illumination. A lamp at 1 m above a desk produces four times the illuminance of the same lamp at 2 m.

Recommended Light Levels by Task

These are the accepted minimum levels for comfortable, safe work. Your first goal is to reach minimums; efficiency optimization comes after.

Space / Activity	Minimum (lux)	Recommended (lux)	Notes
Storage areas, corridors	50	100	Just enough to navigate safely
Stairways	50	150	Consistency more important than brightness
Sleeping areas	10–30	—	Enough to see without disrupting sleep
Living rooms (general)	100	200	Relaxed activities, conversation
Dining areas	100	200	Food should look appetizing
Kitchens (general)	200	300	Food prep safety
Kitchen counter (task)	300	500	Fine cutting, reading recipes
Reading, writing	300	500	Eye strain builds below 200 lux
Drafting, drawing	500	750	Fine lines require good contrast
Woodworking (general)	300	500	Machine operation safety
Woodworking (fine work)	500	1,000	Joinery, carving, measuring
Medical examination	500	1,000+	Wound assessment requires good color rendition
Surgery, dental work	1,000	10,000+	Use directed bright LED arrays
Outdoor security	20–50	100	Movement detection, deterrence

Measuring Light Levels Without Instruments

In the absence of a lux meter, these practical checks give rough assessments:

Hand shadow test: Hold your hand 30 cm above a white surface. If you see a clear, sharp shadow with good contrast, you have at least 200–300 lux. Fuzzy or barely visible shadow: under 100 lux.

Newspaper test: If you can read 10-point newspaper text comfortably at arm’s length, you have approximately 200–300 lux. Squinting or holding the paper closer: under 150 lux.

Counting fingers test: If you can see the outlines of your fingers at arm’s length without difficulty, minimum safety illumination for walking is present (50+ lux).

Smartphone light meter apps: Any Android or iOS smartphone has a light sensor and free lux meter apps that use it accurately enough for planning purposes. Salvage and preserve these for measurements.

Calculating Required LED Output

To hit a target illuminance on a work surface, use this simplified planning calculation:

Required lumens ≈ Target lux × Area (m²) × Utilization factor

Where utilization factor accounts for:
  - How much light misses the target surface
  - Absorption by walls and ceiling
  - Typical range: 0.4–0.7 for rooms, higher for directed task lights

Example — workshop bench 1m × 2m, target 500 lux:

Required lumens ≈ 500 × 2 × 0.5 = 500 lumens at the bench
(using 0.5 utilization because some light goes to walls/ceiling)

But for general workshop safety, you need 300 lux throughout the 5m × 5m space:
Total area = 25 m²
Required lumens ≈ 300 × 25 × 0.5 = 3,750 lumens total
At 100 lm/W (LED), that is 37.5W of LED lighting
At 12V, that is 3.1A — well within a car battery's capacity

Ambient vs Task Lighting Strategy

The most efficient lighting approach splits illumination into two layers:

Ambient (general) lighting: Low-level uniform illumination covering the whole space. Enough to navigate safely, find things, and do undemanding activities. Target 100–150 lux everywhere. This might be achieved with two 5W LED panels in a room.

Task lighting: Bright, directed light aimed at specific work surfaces. A dedicated lamp over the workbench, over the kitchen counter, by the reading chair. 500+ lux concentrated where needed. A single 5W LED in a reflector shade aimed at a bench delivers excellent task illumination.

This approach uses roughly half the total watts of trying to flood an entire space to task levels.

Layered lighting example — 4m × 4m workshop on 12V:

Ambient: 2 × 5W LED panels on ceiling = 10W, ~1,600 lm → 100 lux everywhere
Task: 2 × 5W LEDs in reflectors over benches = 10W → 500+ lux at bench
Security: 1 × 0.5W LED at door = 0.5W → can navigate even if main lights fail
Total: 20.5W, ~1.7A from 12V battery for 4h/night = 82 Wh

Color Rendering and Why It Matters

Lux measures quantity of light. Color rendering index (CRI) measures quality — how accurately colors appear under a light source compared to natural daylight (CRI = 100).

For most survival purposes, CRI is secondary to quantity. However, for specific tasks it matters:

• Medical assessment: Skin color changes (cyanosis, jaundice, infection redness) are harder to see under low-CRI light. Use the highest-CRI LEDs you can find for medical spaces. Warm-white LEDs (2,700K) with CRI 80+ are acceptable; CRI 90+ are ideal.
• Food preparation: Low-CRI light makes meat look grayish, spoiled food look normal, and ripe produce indistinguishable from unripe. Use warm-white high-CRI sources in kitchens.
• Textile work: Color matching in weaving, dyeing, and sewing requires accurate color rendering. Work near a window during the day when possible; use high-CRI LEDs at night.

For workshop, corridor, and storage lighting, CRI matters very little. Cold white (5,000–6,500K) LEDs are fine and often slightly more efficient.

Adapting to Limited Power

When electrical power is severely constrained, these strategies maximize useful illumination:

Reflective surfaces: Whitewash walls and ceiling. A room with white walls reflects 80%+ of incident light. The same room with dark surfaces absorbs 60-70% of the light. Whitewashing costs almost nothing and effectively doubles your usable illumination without additional power.

Mirrors and polished tin: A mirror or polished tin reflector behind a light source redirects otherwise wasted backlight forward. A simple curved tin reflector can triple the useful light reaching a work surface compared to a bare bulb.

Time-of-use control: Do the most demanding tasks during daylight hours through windows. Reserve electric lighting for early morning, evening, and overcast-day tasks. A single well-positioned window on the south side of a workshop (in northern latitudes) provides 10,000+ lux at noon with zero power consumption.

Localized lighting: Instead of lighting whole rooms, light only the person. A single LED task lamp that travels with the worker uses one-tenth the power of full-room illumination.