Spectrophotometry

Accurate Measurement of Optical Properties of Materials

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• Publisher: Academic Press
• Year: 2014
• ISBN:
  9780123860231
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

This volume is an essential handbook for anyone interested in performing the most accurate spectrophotometric or other optical property of materials measurements. The chapter authors were chosen from the leading experts in their respective fields and provide their wisdom and experience in measurements of reflectance, transmittance, absorptance, emittance, diffuse scattering, color, and fluorescence. The book provides the reader with the theoretical underpinning to the methods, the practical issues encountered in real measurements, and numerous examples of important applications.

• Written by the leading international experts from industry, government, and academia
• Written as a handbook, with in depth discussion of the topics
• Focus on making the most accurate and reproducible measurements
• Many practical applications and examples

Chapter 1

Introduction

Thomas A. Germer*; Joanne C. Zwinkels^†; Benjamin K. Tsai* ^* National Institute of Standards and Technology, Gaithersburg, Maryland, USA
^† National Research Council Canada, NRC, Measurement Science and Standards, Ottawa, Ontario, Canada

Abstract

This chapter is an introduction to Spectrophotometry: Accurate Measurements of the Optical Properties of Materials. It presents a short history of spectrophotometry. A very brief description of the principles of uncertainty analysis is given. Finally, an overview of the rest of the book is given.

Keywords

Absorptance

Emittance

Fluorescence

Reflectance

Scattering

Transmittance

Exper. 6. And as these Colours were not changeable by Refractions, so neither were they by Reflexions. For all white, grey, red, yellow, green, blue, violet Bodies, as Paper, Ashes, red Lead, Orpiment, Indico Bise, Gold, Silver, Copper, Grass, blue Flowers, Violets, Bubbles of Water tinged with various Colours, Peacock's Feathers, the Tincture of Lignum Nephriticum, and such-like, in red homogeneal Light appeared totally red, in blue Light totally blue, in green Light totally green, and so of other Colours. In the homogeneal Light of any Colour they all appeared totally of that same Colour, with this only Difference, that some of them reflected that Light more strongly, others more faintly. I never yet found any Body, which by reflecting homogeneal Light could sensibly change its Colour.

Sir Isaac Newton [1]

1.1 Opening Remarks

Spectrophotometry is the quantitative measurement of the interaction of ultraviolet (UV), visible, and infrared (IR) radiation with a material and has an impact on a wide field of science and technology. The nature of this interaction depends upon the physical properties of the material, for example, transparent or opaque, smooth or rough, pure or contaminated, and thin or thick. Thus, spectrophotometric measurements can be used to quantify, in turn, these important physical properties of the material. The choices of spectrophotometric measurements include spectral reflectance, transmittance, absorptance, emittance, scattering, and fluorescence and can be classified as phenomenological optical properties of the material. Spectrophotometric measurements can also be used to probe the intrinsic or internal physical nature of the material, such as its refractive index and extinction coefficient.

The design and performance of optical instruments, ranging from low-cost cell-phone cameras to high-cost microlithography projection tools and satellite telescopes, require knowledge of the optical properties of the components, such as their refractive index, roughness, subsurface scatter, and contamination. The pharmaceutical and chemical industries use optical absorption and fluorescence measurements to quantify concentration, required for accurate dosing and elimination of contaminants. Global climate change simulations require accurate knowledge of the optical properties of materials, gases, and aerosols to calculate the net energy balance of our planet. The properties of thin films, even when they are not intended for optical applications, are often related to their optical reflection, transmission, and scattering properties. Commercial products are often selected by consumers based upon appearance, a complex attribute that encompasses more specific terms, such as color, gloss, and texture. Renewed interest in solar energy has driven the need to maximize the light capture efficiency of solar collectors.

When we are asked to inspect a piece of material, it is our natural inclination to view it by holding it up to a light. The interaction of the light with the material gives us an overall impression of its quality. Our vision is also inherently multispectral, by providing color discrimination on a relatively high spatial resolution. Binocular vision, by allowing us to view the object from multiple directions simultaneously, gives us an ability to perform rudimentary tomography. The spectral, spatial, and directional properties permit us to identify materials, characterize topography, and observe defects, without ever coming into contact with the object. It is not surprising, then, that we seek to make measurements of optical properties of materials in order to better quantify what our own eyes sense qualitatively.

While certain aspects of optics, such as the laws governing refraction of light and the ray nature of light, were well established by the mid-1600s, it was Isaac Newton who discovered that white light was a mixture of colors that could be separated into its components using a prism. It could be argued that Newton performed the first spectrophotometric measurements of this light interaction with a prismatic material. This chapter's epigraph [1] gives an account of his discovery that, in the absence of fluorescence, rays of one color cannot be changed into rays of another, but that different materials simply reflect the colors in different amounts. Newton noted that the color purple was not in the rainbow, but could be created by mixing violet and red rays. He then proposed the basic structure of the color circle and noted that mixtures of any two opposing colors yield a neutral gray.

Newton, of course, used his eyes as the detector. While he could be quite quantitative in measuring angles of refraction, he had more difficulty in estimating intensity or quantifying color. Furthermore, because of his reluctance to accept the wave nature of light, he would never correlate the colors that he observed after dispersion through a prism with the corresponding wavelengths of light. Through the years, however, acceptance of the wave properties took hold, first through the double slit experiment of Young [2] and then through the progressive works of Augustin Fr...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributors
• Volumes in Series
• Preface
• Chapter 1: Introduction
• Chapter 2: Theoretical Concepts in Spectrophotometric Measurements
• Chapter 3: Dispersive Methods
• Chapter 4: Fourier Transform Methods
• Chapter 5: Regular Reflectance and Transmittance
• Chapter 6: Diffuse Reflectance and Transmittance
• Chapter 7: Spectral Fluorescence Measurements
• Chapter 8: Angle-Resolved Diffuse Reflectance and Transmittance
• Chapter 9: Spectral Emissivity Measurements
• Chapter 10: Color and Appearance
• Chapter 11: The Use of Spectrophotometry in the Pharmaceutical Industry
• Chapter 12: Spectrophotometry Applications: Remote Sensing
• Chapter 13: Microspectrophotometry
• Index