Photometry • Illumination • Radiometry • Optics • Imaging • Acoustics

Light, Optics, Imaging & Sound Unit Converter

This converter covers Photometry and Illumination (Lumen, Lux, Candela, Luminance), Radiometry (Watts), Acoustics and Audiometry (dB SPL), Waveforms (Frequency), Spectral and Optics (Wavelength), and Imaging, Display, Visuals and Media (Resolution). Choose a category to see the meaning, the most common formula and real world uses.

Lux: lx = lm/m² Nit: 1 nit = 1 cd/m² dB SPL: 20·log10(p/20µPa) Wave: v = f·λ px/cm = ppi/2.54
How to use

Pick a category, select From and To units, type a value, and the result updates instantly. Use Swap to reverse units and Copy to copy the result.

Why these categories

Light and sound data appears in product specs, labs, photography, display settings, audio testing and engineering reports. Conversions keep values consistent across tools and standards.

Important note

dB types depend on reference. This page supports dB SPL because it has a fixed reference of 20 µPa. Other dB forms (dBm, dBV) require additional context.

Interactive Converter

Category list matches your exact words. Some items are shortcuts (example: Lumen opens Photometry).

Category explanation
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Select a category to see what it means and where it is used.

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Plain language

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Common formula

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Real world uses

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Tip: You can type commas (like 12,500). The converter reads it as 12500.

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Enter a value to convert
Conversion Formula
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Notes: This tool converts units. It does not replace full lighting or acoustics design calculations. For dB conversions outside dB SPL, a reference is required (example: dBm uses 1 mW).

Unit Reference Charts

Quick reference guides for common unit conversions across all categories.

Photometry & Illumination

Unit Symbol Relation
Lumen lm Total light output
Lux lx lm/m²
Foot-candle fc 1 fc ≈ 10.76 lx
Candela cd Luminous intensity
Nit cd/m² Surface brightness

Acoustics & Sound Levels

Source dB SPL Pressure (Pa)
Threshold of hearing 0 dB 20 µPa
Normal conversation 60 dB 0.002 Pa
Busy traffic 80 dB 0.02 Pa
Threshold of pain 130 dB 6.3 Pa
Jet engine 140 dB 20 Pa

Frequency Bands

Band Range Application
Infrasound < 20 Hz Seismic, vibration
Audio 20 Hz - 20 kHz Human hearing
Ultrasound 20 kHz - 1 MHz Medical, cleaning
RF 3 kHz - 300 GHz Wireless, broadcast
Audio CD 44.1 kHz Standard sampling

Electromagnetic Spectrum

Type Wavelength Frequency
Radio mm - km kHz - MHz
Microwave 1 mm - 1 cm GHz
Infrared 700 nm - 1 mm THz
Visible 400 - 700 nm 400 - 800 THz
UV 10 - 400 nm PHz

Resolution & Pixel Density

Device PPI px/cm
Standard print 72 ppi 28.3 px/cm
Photo quality 300 ppi 118.1 px/cm
Smartphone 326 ppi 128.3 px/cm
Retina display 458 ppi 180.3 px/cm
4K monitor 163 ppi 64.2 px/cm

Quick Conversion Factors

1 foot-candle 10.764 lux
1 kilolux 92.903 foot-candles
1 nit 1 cd/m²
1 stilb 10,000 nits
1 bel 10 decibels

Worked Examples

These examples are pre computed so visitors can sanity check results and learn common magnitudes.

Examples are computed on page load

Example: Illumination

Convert 500 lux to foot-candles.

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Example: Luminance

Convert 300 nits to cd/m² (same unit) and stilb.

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Example: Acoustics (dB SPL)

Convert 1 Pa to dB SPL (ref 20 µPa).

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Example: Frequency

Convert 44.1 kHz to Hz.

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Example: Wavelength

Convert 550 nm to µm.

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Example: Resolution

Convert 300 ppi to px/cm.

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Visual Learning Guides

Understand the relationships and scales of light, sound, and electromagnetic properties.

Illumination Scale (Lux)

Starlight 0.0001 lx
Moonlight 0.25 lx
Office (typical) 500 lx
Sunny day (outdoor) 100,000 lx

Sound Pressure Level (dB SPL)

Threshold of hearing 0 dB
Whisper 30 dB
Normal conversation 60 dB
Threshold of pain 130 dB

Visible Light Spectrum

Violet: 380 nm Red: 700 nm

The visible spectrum ranges from approximately 380 nm (violet) to 700 nm (red). This represents the wavelengths that the human eye can detect.

Frequency-Wavelength Relationship

Relationship: v = f × λ

For light in vacuum: c = 3×10⁸ m/s

10 Hz 30,000,000 m
1 kHz 300,000 m
1 MHz 300 m
500 MHz 0.6 m

FAQ

What is the difference between lumen, lux, candela and luminance?

Lumen (lm) is total light output. Lux (lx) is illuminance on a surface (lx = lm/m²). Candela (cd) is intensity in a direction. Luminance (cd/m²) is surface brightness (nits).

Why do some categories repeat the same converter?

Some words are "shortcuts" to the same units. For example, Lumen opens Photometry, and Display opens Resolution. This keeps your category names exactly as visitors search.

What is dB SPL?

dB SPL is sound pressure level referenced to 20 µPa: dB SPL = 20·log10(p/20µPa).

How are frequency and wavelength related?

They are linked by wave speed: v = f·λ. For light in vacuum, f = c/λ.

What wavelength is visible light?

Visible light ranges from approximately 380 nm (violet) to 700 nm (red). The peak sensitivity of the human eye is around 555 nm (green-yellow).

What is the difference between PPI and DPI?

PPI (pixels per inch) is used for screens and digital displays. DPI (dots per inch) is used for printers. They measure the same concept but apply to different devices.