MICROSCOPIC FIBER IDENTIFICATION BY REFRACTIVE INDEX

Mark A. Goodman

Determining the identity of a fiber can sometimes be difficult. A method that is useful in determining the fiber category by refractive index is presented. Application to distinction between hemp and linen is discussed.

Refractive Index

There is a common misconception that the speed of light is constant. That's right; the speed of light is only a constant in a vacuum or any other homogeneous material. Light from a microscope enters a sample the speed of light is slowed down. The common definition of index of refraction (n) for a fiber is:

For fibers the refractive index is related to the internal (molecular) structure and is independent of factors which determine its outer (morphological) features.

Anisotropy

In Figure 1 the longitudinal and perpendicular in which light can travel through a fiber is shown. For an anisotropic fiber the speed of light across the fiber (perpendicular) and the speed of light along the fiber (longitudianal) will not be identical. Identification of fibers can be made based on this difference using refractive index. The longitudinal refractive index is designated n and the perpendicular refractive index n.

Figure 1

The refractive index depends strongly upon the wavelength of light. Therefore the refractive index measurements are measured using light of a wavelength of 589 nanometers, referred to as the Fraunhofer D line or Sodium D Line. This can be accomplished by using a Roscoe #23 Orange filter (See Appendix 1).

Refractive Index of Fibers

BECKE LINE

A band or halo of light (due to diffraction/refraction) can be seen at the edge of a fiber when the refractive indices of the fiber and its mounting medium are different. In practice, the Becke Line is produced by focusing above and below the plane of best focus but is not discernable when the fiber is in focus. This method is named after the Austrian geologist and petrologist, Friedrich Johann Karl Becke, 1855-1931, who devised the method.

The Becke Line always moves toward the material of higher refractive index on focusing above the plane of best focus.

The Becke Line moved inward and outward are presented. Usually it is best to observe the Becke line at a total magnification of 200X to 500X.

Becke Line Moved Inward (Shown in White)

Becke Line Moved Outward (Shown in White)

DETERMINATION OF PARALLEL AND PERPENDICULAR REFRACTIVE INDICES

MICROSCOPE SET UP

Only a linear polarizer and Roscoe #23 filter (Rosco Laboratories Inc., 52 Harbor View, Stamford, CT 06902) are need to modify a standard light microscope for Becke Line analysis.. The polarizer is relatively inexpensive and can be purchased at a camera store. Make sure that the polarizer is a linear polarizer. The microscope set is shown in Figure 5.

Figure 5: Microscope Set Up Diagram

The polarizer must be oriented such that the polarizer direction runs in the east-west direction relative to the front of the microscope (Figure 6). See Appendix 1 for determination of the polarizer direction. If you are using a polarizer light microscope, the polarizer should be already oriented correctly. A discussion of polarization of light and polarizer orientation is presented in Appendix 2.

Figure 6: Polarizer Orientation

FIBER ORIENTATION ON SLIDE

For determination of the parallel refractive index (n), the fiber must be oriented in the parallel (east-west) direction when viewed in the microscope. In other words the polarization direction of the light and fiber orientation must be the same as shown below and as discussed in Appendix 2. The polarization direction and fiber need to be perpendicular for obtaining the perpendicular refractive index of the fiber (n).

Orientation of Fiber on Slide – as viewed in the microscope

REFRACTIVE INDEX IMMERSION LIQUIDS AND THE BECKE LINE

The iris diaphragm needs to be closed until is it just slightly larger than the microscopes field of view. The distance between the objective and the fiber sample is reduced (see figure below) using

the fine focus knob of the microscope. If the Becke Line moves inward, the fiber has a lower refractive index than the immersion liquid and is assigned a negative sign (-). If the Becke Line moves outward from the fiber, the immersion liquid has the higher refractive index than the fiber and is assigned a positive sign (+).

FIBER IDENTIFICATION

It is recommend that for fiber identification that an immersion liquid of refractive index 1.550 be used initially. Refractive index immersion liquids can be purchased from Cargille Laboratories, 55 Commerce Rd., Cedar Grove, NJ 07009 USA, (973)-239-6633, cargillelabs@aol.com.

A few fibers are placed on a clean microscope slide and a drop or two of the immersion liquid is added followed by placing a cover slip on top. Examine the slide on the microscope and observe the fiber in the parallel orientation, focusing on a sharp edge of the fiber. The iris diaphragm needs to be closed until is it just slightly larger than the microscopes field of view.

Determine if the immersion liquid has a higher or lower refractive index then the fiber by observing the Becke line as discussed above. The process can be repeated using other immersion liquids until the n of the fiber has been located within an acceptably small range on a trial and error basis. The fiber is then oriented in the perpendicular direction and the process is repeated until n is determined.

The Fiber Identification table below has the n and n of several common fibers. The fibers are classified from low to high birefringence. Birefringence is defined below:

birefringence = n - n

STANDORT PLOT

The Standort Plot shown below is useful in determining different basic types of fibers. For example to know if a fiber is polyester, the Standort Plot shows that the parallel refractive index (n) is greater than 1.60. Since there are no other fiber classes that have a refractive index that high. An immersion liquid of refractive index 1.550 could be used and if the n is determined to be greater than the immersion liquid and then the fiber is identified as polyester. This can be done without performing the n. This is a basic example similar schemes could be used to perform the fiber identity for other fiber classes.

Linen and Hemp Fiber Identification

The Becke Line can be used to determine if a fiber is linen or hemp. Refractive indexes for some fibers presented below.

In most cases ramie can be distinguished from linen or hemp based on the fact that ramie has a readily identifiable twist which is not present in hemp or linen fibers. Based on the n values presented in table, it would be impossible to tell the difference between ramie, jute, linen, or hemp.

Fiber n n

Wool 1.553-1.556 1.542-1.547

Silk 1.591-1.595 1.538-1.543

Cotton 1.573-1.581 1.529-1.534

Ramie 1.595-1.599 1.527-1.540

Linen 1.594-1.596 1.528 -1.532

Hemp 1.585-1.591 1.526-1.530

Jute 1.577 1.536

Viscose rayon 1.539-1.550 1.514-1.523

Cellulose acetate 1.476-1.478 1.470-1473

Polyethylene 1.556 1.512

Refractive indices of fibers using immersion oils can be determined to within 0.002. Since this value is than the reported difference between hemp and linen use of refractive index method was investigated.

Initial investigation revealed that the published values for the refractive index of hemp and linen given above were inaccurate. From this investigation the refractive index of linen (n) was ~1,579 while that of hemp was ~1.582.

The refractive index data for different sources of hemp and linen are presented below. Based on this data the identification hemp or linen based on refractive index is possible using the Becke Line method for determination of refractive index.

Parallel Refractive Index Data for Different Sources of Hemp and Linen

SampleRefractive Index Oil

Comments1.576 1.578 1.580 1.584

Line

n

Raw & Unprocessed + + - Using Polarized Light Microscope

Raw & Unprocessed + + - Using Polarized Light Microscope

Raw Linen - Unprocessed + + - Standard Microscope using Nikon Polarizer

Linen Suit Collar 1 + + - Using Polarized Light Microscope

cloth blended with cashmere + + - Using Polarized Light Microscope

dry spun yarn + + - Using Polarized Light Microscope

blue yarn from Belgium + + - Using Polarized Light Microscope

wet spun yarn + + - Using Polarized Light Microscope. Slight Becke line

Jones NY collar - made in China + + - Using Polarized Light Microscope

Bleached Linen + + - Using Polarized Light Microscope

White Linen Cord + + -

Hemp Cord – Mislabeled + + -

He

mp

Hemp Cord Source 1 + + + - Using Polarized Light Microscope

Green Hemp Cord + + + - Using Polarized Light Microscope

Hemp Cord Source 2 + + + - Using Polarized Light Microscope

Hemp/Wool Sweater + + + - Using Polarized Light Microscope

72% Wool/28% Hemp cloth + + + - Using Polarized Light Microscope

Hemp Rope 1 (Mystic Seaport) + + - Using Polarized Light Microscope

Hemp Rope 2 (Mystic Seaport) + + - Using Polarized Light Microscope

APPENDIX 1: Determination of Polarizer Orientation.

To determine the orientation of a polarizer hold the polarizer in your hand and observe a reflect spot on a glass surface or a shiny black (non-metallic) flat surface. Reflected light from these surfaces can produce nearly polarized light. While observing reflection turn the polarizer slowly until the you observe a spot in which the light is reduced to a minimum or is extinct. This will mean that the orientation of the polarizer will be from the top to bottom. You can usually place a small mark at the top and bottom to indicate orientation direction.

APPENDIX 2: Polarization of Light

The light source of a microscope produces unpolarized light which is made up of light waves vibrating in all directions. When unpolarized light passes through a polarizer, only light in the polarization direction passes trough. This is diagrammatically shown below. The incident beam is made of light waves in every 360 degree orientation. The polarized light shown exiting the polarizer in only in one direction called the polarization direction.

The microscope from the front view is shown below. When the polarizer is oriented in the east-west direction (in the plane of the paper) the polarized light emerging from the polarizer, represented in the blue box is also in the plane of the paper. This is important, because the polarization direction and fiber orientation must be the same for determination of the parallel refractive index.

Figure 8: Front View of Microscope with Polarized Light Orientation

APPENDIX 3 – Use of Filter for Refractive Index Measurements.

The light source in a microscope is generated from a bulb which produces white light. This light is made up of a mixture of wavelengths in the visible spectrum. In the refractive index depends strongly upon the wavelength of light. In other words small changes in wavelength will cause large changes in the refractive index (see figure below). Refractive index measurements are performed at a specified wavelength. The most common measurement wavelength is the Fraunhofer D line at 589 nanometers. The Fraunhofer F, D and C lines are shown in the figure.

The importance of acquiring refractive index at a known wavelength is graphically demonstrated in the figure below. The graph shows refractive index of two fibers (blue and red lines) which have different dependences on wavelength. At the Fraunhofer D wavelength the fibers have the same refractive index of 1.50. A small change in wavelength results will change the refractive index of the fiber. A wavelength change from 589 nm to 600 nm results in a refractive index of 1.498 for the blue fiber and the red fiber will have a refractive index of 1.496. This demonstrates that accurate refractive index measurements require a filter to insure that the wavelength does not varry. Optical filters are usually very expensive. An inexpensive solution is to use Roscoe #23 Orange filter (Rosco Laboratories Inc., 52 Harbor View, Stamford, CT 06902) which will provide light equivalent to the Fraunhoffer D line. The importance of using a filter when obtaining Becke lines for refractive index measures was reported by R.C. Emmons and R.M. Gates, American Mineralogist. 1948; Vol:33, 612.

Dependence of Refractive Index on Wavelength of Light for Two Fibers.