When selecting LEDs or designing a new luminaire, the terminology can quickly become overwhelming. Many buyers and even junior engineers often ask: "Why does my LED have a high lumen rating, but the ground still looks dark?" or "Why is this fixture so glaring, yet it doesn't illuminate the workspace properly?"
The root of this confusion lies in mixing up four entirely different dimensions of light: how much light is emitted, how it is concentrated in space, how much lands on a surface, and how the human eye perceives it. Understanding these basic terms is not just academic—it is the critical first step in making the right decisions for luminaire design and secondary optical lens selection.
What is Luminous Flux? & What is Lumen?
The Definition: Luminous Flux is the total amount of visible light energy emitted by a light source in all directions per unit of time. Lumen (lm) is simply the standard unit of measurement for luminous flux. When you read an LED specification sheet, the lumen value tells you the absolute maximum light output that the bare LED chip can produce.
The Water Analogy: Think of a showerhead or a water hose. Luminous flux is equivalent to the total volume of water flowing out of the nozzle every second. It does not matter if the water is spraying widely everywhere or focused into a tight, narrow stream; the total amount of water (the lumens) released by the source remains exactly the same.
The Lens Connection (Why it matters for design) : This brings us to a crucial rule in optical engineering: Secondary lenses do not create lumens. The lumen output is the factory setting of your chosen LED chip. In fact, due to the laws of physics, passing light through any material (even the highest-grade PC or PMMA) results in a minor transmission loss.
If you leave a bare LED without a lens, those raw lumens will scatter uselessly in a nearly 180-degree or 360-degree sphere. The primary job of an Asahi Optics custom lens is to efficiently collect and manage every single available lumen.
What is Luminous Intensity?
The Definition: While luminous flux measures the total light emitted, Luminous Intensity measures the power of that light traveling in a specific direction. Its standard unit of measurement is the Candela (cd) .
The Water Analogy: Imagine you take that same showerhead and twist the dial from a wide, gentle spray to a narrow, high-pressure massage jet. The total volume of water (lumens) hasn't changed at all. However, because you forced all that water into a single, tight direction, the pressure or force in that specific direction is now incredibly high. That concentrated force is your luminous intensity (Candela) .
The Lens Connection (Why it matters for design) : This is exactly where Asahi Optics' custom lenses perform their magic. A bare LED chip throws light everywhere, meaning its luminous intensity in any single direction is relatively low. By designing a custom Total Internal Reflection (TIR) lens or a precision reflector, we gather those scattered lumens and squeeze them into a tightly controlled beam (for example, a 15° spotlight). This drastically increases the Center Beam Candlepower (CBCP) , giving the light the penetrating power needed to reach high ceilings or highlight specific architectural details.
What is Illuminance? & What is Lux?
The Definition: Illuminance is the measure of how much luminous flux is actually landing on a specific target surface area (like a desk, a floor, or a roadway). The standard unit for illuminance is Lux (lx) , where $1 \text{ lx} = 1 \text{ lm/m}^2$.
The Water Analogy: If lumens are the total water leaving the hose, and intensity is the pressure of the spray, then Lux is the depth of the water puddle that actually forms on the ground. If you spray the water over a large driveway, the puddle is very shallow (low Lux). If you spray it all into a small bucket, the water gets deep very quickly (high Lux).
The Lens Connection (Why it matters for design) : When clients evaluate a lighting project, they usually don't care how many lumens the fixture has; they care whether the desk or the road meets the required Lux standard (e.g., 500 Lux for an office). Without the right optic, a powerful LED might create a harsh hotspot (too much Lux in the center) and leave the edges dark. We design specialized light distributions—such as linear or extrusion lenses for indoor lights—to spread the light evenly, ensuring a uniform Lux level across the entire target area.
What is Luminance?
The Definition: Luminance is the only concept here that deals with human perception. It is the amount of light that either reflects off a surface into your eyes, or the absolute brightness you see when looking directly at the light source itself. It is measured in Candelas per square meter (cd/m²) , often referred to as Nits.
The simplest way to remember the difference: Lux is the light arriving at the table. Luminance is the light leaving the table (or the lamp) and hitting your eye.
The Water Analogy: Luminance is how hard the water splashes back into your face when it hits the ground, or how blinding it is if someone points the high-pressure hose directly at your eyes.
The Lens Connection (Why it matters for design) : If the luminous surface of a luminaire has extremely high luminance, it causes Glare—a severe problem that leads to visual discomfort and fatigue. This is a critical factor in Human-Centric Lighting. At Asahi Optics, we use micro-structures, frosted surfaces, or precision cut-off angles (to control the Unified Glare Rating (UGR) ) in our custom lenses. These designs diffuse the concentrated light source, significantly lowering the surface luminance so the fixture is comfortable to look at, all while maintaining the necessary Lux on the workspace below.
Conclusion: Why Do These Matter for Custom Lens Design?
An exceptional, high-lumen LED chip is only half the battle. Without precision optics, your luminous intensity will be weak, your illuminance (Lux) will be uneven, and your surface luminance will be glaringly uncomfortable.
Secondary optics are the critical bridge between raw diode power and actual human visual comfort. When you are planning your next luminaire project, don't just look at the bare LED specifications. Bring your true project requirements—your target Lux, desired beam angle, and anti-glare (UGR) needs—to Asahi Optics. Our optical engineering team will design and manufacture the perfect custom lens to put exactly the right amount of light, exactly where you need it.
