VISIBLE LIGHT METER – FULL DESCRIPTION

A Visible Light Meter (also called Lux Meter or Illuminance Meter) is an instrument used to measure the intensity of visible light in an environment. In NDT, especially in Fluorescent Penetrant Inspection (FPI) and Fluorescent MPI, it is used to ensure that ambient white light levels meet the required standards for proper defect visibility.

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✔ What a Visible Light Meter Measures

A visible light meter measures illuminance, represented in:

• Lux (lx) – lumens per square meter

• Foot-candles (fc) – lumens per square foot (1 fc = 10.76 lux)

This measurement ensures that the environment has adequate or allowable visible light for inspection tasks.

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✔ Purpose in NDT

A visible light meter is essential to:

• Measure ambient visible light in the inspection area

• Check compliance with standards like ASTM E1444, ASTM E1417, ISO 3059

• Ensure the darkroom conditions meet requirements:

  • Maximum visible light for fluorescence inspection: 20 lux (2 fc)

  • Minimum visible light for post-inspection: 1000 lux (100 fc)

• Verify inspector eye adaptation conditions

• Evaluate the lighting quality for both UV inspection and white-light inspection

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✔ Construction and Components

1. Light Sensor (Photodiode)

• Silicon photodiode

• Sensitive to the visible spectrum (380–780 nm)

• Sometimes includes color-correction filters

2. Optical Filter

• Mimics human eye response curve (V(λ) function)

• Ensures accurate illuminance measurement

3. Display Unit

• Shows illuminance in lux or foot-candles

• Digital meters show numeric values

• Analog meters use a needle pointer

4. Electronics

• Converts electrical signal from sensor into calibrated lux/FC readings

5. Power Source

• Battery powered (AA/AAA or rechargeable)

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✔ Types of Visible Light Meters

1. Digital Lux Meter

• High accuracy

• Portable

• Backlit display

• Data logging ability

2. Analog Lux Meter

• Simple and economical

• No digital electronics

3. Advanced Photometric Meters

• Can measure color temperature

• Data storage and wireless transfer

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✔ Typical Technical Specifications

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✔ How to Use a Visible Light Meter

1. Place the sensor horizontally at the inspection location.

2. Ensure no shadows or obstructions interfere.

3. Hold the meter at the inspector’s eye level.

4. Read the displayed illuminance value.

5. Compare with required NDT standards:

   • Inspection under UV-A: Max 20 lux ambient light

   • Visible-light inspection: Min 1000 lux illumination

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✔ Applications

• Fluorescent Penetrant Inspection (FPI)

• Fluorescent Magnetic Particle Inspection (FMPI)

• General lighting surveys

• Workplace safety compliance

• Industrial and laboratory lighting evaluation

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✔ Advantages

• Accurate and fast measurements

• Easy to use

• Essential for meeting NDT standards

• Portable and lightweight

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✔ Limitations

• Needs periodic calibration

• Sensitive to dust and scratches on sensor

• Incorrect angle or positioning may affect readings

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