Two LED lamps can each draw exactly 8 watts of power, yet still produce significantly different lighting effects. One lamp might cast a gentle illumination into a room. The other might produce a strong spot of bright light on your desk. This is the reason why wattage cannot be the sole criteria to consider when evaluating whether a lamp is suitable for a reading chair, worktop, hallway or an ambient layer.
Wattage is a measure of how much electrical energy is required to power a fixture. Lumens represent the total amount of visible light emitted by a fixture, or a source of light in general. Lux (also sometimes referred to as foot-candles) indicates the strength of illumination on a specific surface.
If you are curious, try this simple experiment: Position a lamp at a movable location in front of a table with a book or a sheet of plain white paper on it. From different distances, look at the page. Notice the changes in the illumination as you move the lamp closer or further from the surface. The lumens have not changed. The wattage has not changed. The beam has not changed. The illumination has changed because you changed the distance of the light source from the surface. Tilt the lamp or use a lamp that has a wider spread, and the illumination spreads out and becomes dimmer. This demonstrates why lux measurements are dependent on the distance, beam angle, direction and size of the illuminated surface area.
Lumens are useful for comparing the total light output of different fixtures. In general, a lamp that is rated at a higher lumens value will produce a brighter, more powerful output. However, a higher lumens value does not necessarily result in a better light. A high-lumen light source, for example, with a wide diffuser and opaque shade, can still provide soft and welcoming ambient lighting. The same lumens produced through a focused source with a small beam can produce a powerful spot with high contrast on a dark surrounding area. Luminance values can also differ based on design choices. Light that gets directed away from a room by a lamp’s shade, lens, reflector or a recessed fixture, is not being utilized.
Lux values are important to consider if there is a defined surface plane to be lit, like a desk, a countertop, a mirror, the floor or a wall. You can use a lux meter or an approximate lux meter app to compare these surfaces in the room you are working with. (Note that a smartphone app for these measurements is not considered a professional-grade instrument; use it as a comparative reference tool rather than for precise measurements.) Make sure you hold the meter at the same angle and the same distance for each measurement. Record the results as you test the readings of a lamp at various angles, distances and when the light is reflected off of a wall. You will notice the lux readings will vary with each new position. The lamp has not changed. The lux levels change as your testing positions change.
There is often confusion between an old incandescent lamp that has been replaced with an LED lamp. Many of us were taught by the wattage to measure the power of a light bulb. So, naturally, many people assume that the higher the wattage, the brighter the light will be. With modern LED lamps, wattage does not necessarily correspond to lumen output. Even with the equivalent wattage listed for many lamp boxes, this is usually based on the wattage of older lamp types. The best thing to do is to look at the lumen values for comparison first. Then, pay attention to the beam angle, color temperature, CRI and the type of diffusion for each lamp. These factors will help you understand why one lamp might produce a very bright, direct beam, while another lamp may offer indirect, or diffused illumination.
Before making a lamp selection, be very clear about what you want the light to do. For general lighting in a room, you can look at the total luminance or lumens for the fixture. In general terms, it does not matter which way you distribute that light, as long as it has adequate brightness to provide a welcoming glow for most tasks. For reading or for specific task lighting on a desk or worktop, it is important to understand the total illuminance on those surfaces. You will want to pay attention to the direction of the light as well to avoid glare or hand shadows. For accent or decorative lighting, look carefully at beam angles and how much contrast your lights produce. This is the time you do not necessarily want the highest lumens value possible. Finally, always ask the three most important questions: How much light does the lamp produce? How much energy does the light consume? How much light is reaching the surface where it is needed?