Chapter 2: Crime Scene Bloodstain Pattern Analysis

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Welcome back to the Deep Dive.

Today we're jumping into something you see a lot in crime shows but maybe don't fully grasp the science behind it.

Blood stain pattern analysis.

BPA.

That's right.

It sounds complex, maybe even a bit grim, but it's a really powerful tool.

So we're going to take a deep dive into the core principles, drawing from a foundational text, and really break down the physics and the methods for you.

Make it visual even without the visuals.

Exactly.

BPA is at its heart applying science, physics, biology, geometry to look at blood stains, their shape, where they are, in what order they appeared.

It all tells a story.

It's about reading that story, right?

What's the main goal when an analyst looks at these patterns?

What are they trying to figure out?

The mission is really reconstruction.

You want to know the sequence of events.

What happened first?

Where was the blood source, roughly?

Where were the people involved positioned?

How many times did an impact occur?

Things like that.

It helps investigators understand if it looks like an accident, a homicide, maybe even a suicide.

That's a huge amount of information from just

stains.

So where do we start, I guess, with the blood itself, its properties?

Absolutely.

You have to start there.

Blood makes up what?

About 8 % of an adult's weight, and it's mostly water, plasma, but also has cells, proteins, salts.

Density -wise, it's just a little heavier than water, but the key things are viscosity and surface tension.

Viscosity.

That's thickness, right?

How much it resists flowing.

Yeah, basically.

And blood is quite viscous, maybe five times more than water, but surface tension, that's really important.

The molecules in the blood want to stick together.

That's cohesion.

This makes a drop try to pull itself into the smallest possible surface area.

So picture a drop hanging there before it falls.

It forms that sphere, that kind of pendant shape.

Because of the surface tension holding it together.

Exactly.

Then gravity wins, it detaches, and air resistance plays a role as it falls.

But when it hits something,

that perfect sphere idea probably goes out the window depending on what it hits.

Oh, completely.

The target surface is a massive factor.

If it lands on something smooth, non -porous, like say glass or smooth tile, you'll get a relatively clean circular or maybe elliptical stain.

But if it hits something rough or porous, think concrete, wood, fabric, the texture breaks up that nice shape.

The edges get distorted,

spiky.

We call it spiking or splining.

And analysts have to account for that distortion.

Definitely.

Because the shape is what we measure.

If the shape's distorted by the surface, it throws off the calculations we need to make later.

Right.

Okay, so that's the physics.

But what about finding the blood in the first place?

Sometimes it's cleaned up or diluted, right?

Not always obvious.

Correct.

That's where chemical enhancement is vital.

We use very sensitive chemicals that with components of blood, even if it's latent or heavily diluted.

Making the invisible visible.

Pretty much.

Often they cause chemiluminescence, a glow or fluorescence under specific light.

Like luminol.

It's the famous one, the blue glow you see in documentaries.

Luminol is great for searching large areas quickly, finding potential spots.

But it's not specific.

It can react with other things.

Then you have presumptive tests.

Phenolphthalein, LMG, TMB.

And those tell you it's likely blood.

Exactly.

A positive reaction there gives you a strong indication you're dealing with actual blood, which lets you proceed with more confidence.

And once you find it, the next step is critical documentation.

Right.

You can't just eyeball it.

The text emphasizes meticulous notes, sketches, and especially photography.

Absolutely non -negotiable.

And the photography has very specific rules, because again, we're using these photos for measurements.

Okay.

So what are the rules?

What kind of photos?

You need three main views.

First, the overall view.

That shows the whole scene, or a large part of it, giving context.

Where's this pattern in the room?

Big picture first.

Then you zoom in a bit for the mid -range photograph.

This focuses on a specific area, like one wall section or a piece of furniture.

You can see individual stains within the pattern, but still see how they relate to each other.

Got it.

And finally, the close -up photograph.

This is usually done with a macro lens, focusing on single, well -formed stains.

These are the money shots for analysis, because they give us the detailed shape we need to measure.

And what about making sure those measurements are accurate from the photo?

Two golden rules.

One,

always include a scale of measurement right next to the stain in the photo, like a small ruler.

Do you know the actual size?

Yep.

And two, critically,

the camera lens must be parallel to the surface the stain is on, straight on.

Why parallel?

If you tilt the camera, even a little, you create perspective distortion, parallax.

It stretches or squashes the image of the stain.

Then your width and length measurements are wrong, and all the math based on them will be wrong, too.

Okay, that makes total sense.

Precision from the very start.

So you found it, you photographed it perfectly.

Now, analyzing the spatter stains, the ones made by force.

Right.

Spatter happens when some external force hits liquid blood.

The key thing here is that the amount of force affects the size of the droplets created.

More force, smaller drops.

Generally, yes.

Higher energy overcomes the blood's surface tension more effectively, breaking it into smaller pieces.

We classify spatter based on impact velocity, which correlates strongly with stain size.

Okay, break down those velocities for us.

Sure.

First is low velocity impact spatter.

This is from forces traveling less than,

say, 1 .5 meters per second.

The resulting stains are usually quite large, over 4 millimeters in diameter.

Think blood dripping under gravity or maybe very slow movement.

Just falling or maybe a light cast off.

Right.

Then medium velocity impacts spatter.

This is typically associated with things like beatings, stabbings, more force involved.

The stains are smaller, usually in the 1 to 4 millimeter range.

Okay, so size is already telling you something about the web interaction.

And finally, high velocity impacts spatter.

This involves extreme force like gunshots, explosions, maybe some industrial accidents.

The stains are tiny, usually less than 1 millimeter.

Often looks like a fine mist or spray.

Wow.

Okay, so the size points to the type of event.

What about direction?

How do you know which way the blood was going?

That comes from the shape.

When a drop hits a surface at an angle, not straight down at 90 degrees, it elongates.

It forms an ellipse, the parent stain.

And the shape of that ellipse tells you the direction.

Yes.

As the drop hits and moves forward, the momentum pushes the back end out.

This creates a feature we call the spine, or sometimes a tail.

It's the pointed edge, and it always points in the direction the blood droplet was traveling.

Like an arrow built into the stain.

Exactly.

And sometimes you'll see tiny satellite stains that break off from the main drop on impact.

They usually land just ahead of the parent stain, also indicating direction.

Okay, so shape gives direction.

And I remember the core math bit is figuring out the angle it hit the surface, right?

The angle of impact.

Yes.

Angle of impact, usually called alpha.

And this is where that elongation comes back in.

It's simple trigonometry, actually.

Simple for you, maybe.

How does it work?

Well, the more elongated the stain, the shallower the angle it hit at.

We measure the width, W, of the elliptical part of the stain and its length, L.

The sine of the impact angle is just the width divided by the length.

So once in alpha equals W.

Ah.

Okay, so if width equals length, it's a circle.

W is one, sine is one, angle is 90 degrees.

Straight down.

Perfect.

And if it's really long and thin, the width is much smaller than the length.

W is small, the sine is small, so the angle is very low, very shallow.

But given what you said about surface distortion,

getting accurate width and length must be tricky sometimes.

It absolutely is.

That's why analysts have to be selective.

You choose the best formed stains, the ones least affected by texture, and you usually measure several, maybe 8 to 12 stains in a pattern to get a reliable average angle.

Safety in numbers.

Okay.

Okay, so you have the direction and the angle for several stains.

How do you pinpoint where the blood actually came from in 3D space, the area of origin?

Two main methods.

The one people often see is the string method.

It's very visual.

Yeah, with all the strings going back from the wall.

That's the one.

You calculate the impact angle for each chosen stain, then you attach a string to the stain, run it back away from the surface at that exact angle using a protractor, where all the strings from the different stains come together, where they converge in space.

That's your estimated area of origin.

Looks dramatic, but I imagine it could get crowded with strings.

It can, especially if you have many stains in a small area.

The other method is purely mathematical, the tangent method.

Okay, how does that work?

No strings attached.

Right.

First, you figure out the 2D area of convergence on the surface itself.

You just draw straight lines back through the long axis of each selected stain.

Where those lines cross on the wall or floor, that's your convergence point.

Okay, a 2D point on the surface.

Then for each stain, you measure the distance D from the stain to that convergence point along the surface.

You already calculated the impact angle.

The height of the origin above that convergence point is just H equals D times the tangent of alpha.

H equals D, D1 ALU, phi at tan alpha.

Ah, using the tangent function.

So you calculate the height for several stains and average them.

Typically, yes.

That gives you the third dimension, the height off the surface, locating the area of origin in 3D space.

That's amazing, taking those tiny shapes and angles and projecting back to a point in space.

Okay, let's shift gears slightly and talk about the broad categories of patterns.

The text mentions three, passive, transfer, and projected.

Right, those are the main classifications.

Passive blood stains are formed primarily by gravity, no significant external force other than what caused the initial bleeding.

Like drips.

Exactly.

Drip stains, or if the source is moving while dripping, a drip trail.

If blood accumulates on a surface, that's a pool.

If it runs down a vertical surface, that's a flow pattern.

Simple enough.

Anything tricky about passive stains?

You mentioned a bubble ring.

Yeah, sometimes air gets trapped in a pool of blood and as it dries, it can leave a distinct bubble ring pattern.

Also, important point, if a drop falls into an existing pool of blood, it can splash and create small secondary spatter stains around the pool.

So a passive event can generate something that looks like projected spatter.

Ah, okay, context is key.

What about transfer blood stains?

Transfer is all about contact.

A bloody object touches a non -bloody surface, or vice versa.

The key here is differentiating swipe versus wipe.

What's the difference?

It's about timing and motion.

A swipe pattern is when a bloody object moves across a surface, depositing blood as it goes.

Think of a bloody handprint being dragged or a shoe print.

Blood is transferred during the motion.

Okay, so a wipe is different how?

A wipe pattern happens when an object moves through an existing wet blood stain.

So the blood was already there and something disturbed it.

This alters the original stain.

So a wipe tells you the blood was there first.

That seems important for sequencing.

Hugely important.

And related to wipes is something called a perimeter stain.

This happens if a stain is wiped after it has started to dry.

The edges might have dried enough to remain while the wetter center gets wiped away.

Seeing that perimeter gives you clues about the timing.

How long the blood might have been there before being disturbed.

Fascinating.

Okay, last category,

projected blood stains.

This is where the force comes in, right?

More than just gravity.

Definitely.

This includes the impact patterns we talked about earlier.

Low, medium, and high velocity spatter from something hitting liquid blood.

It also includes cast -off patterns.

Cast -off.

That's blood flung from a moving object that's already bloody.

Typically, think of a weapon being swung back or forward between blows.

It creates linear or arc -shaped patterns.

Right.

And gunshot spatter is specific too.

Yes.

With gunshots, you often get both forward spatter, which travels in the same direction as the bullet, usually from an exit wound, and back spatter or blowback, which travels back towards the firearm, usually from an entry wound.

Back spatter tends to be finer.

What about patterns from internal injuries?

Good question.

You can get an expiration pattern.

This is blood forced out of the nose, mouth, or a chest wound by airflow breathing or coughing.

It often looks like a fine spray, sometimes with small air bubbles mixed in because it's mixed with saliva or mucus, indicates airway involvement.

Distinctive.

And the really dramatic one, arterial spurting.

Ah, yes.

The arterial spurt pattern.

This is caused by blood exiting the body under pressure from a breached artery.

It's usually very recognizable, large volume stains, often in an arcing pattern up a wall, reflecting the rise and fall of blood pressure with the heartbeat.

Big spurts correspond to the systolic pressure.

Wow.

And one more.

Sometimes the lack of blood is important.

The void.

The void pattern.

Absolutely critical.

This is an area within a spatter pattern where there are no stains, but the pattern continues all around it.

It means something.

An object or even a person was physically blocking the blood spatter at that location when the event happened.

Finding a void tells you where something was.

So putting it all together, we've gone from the basic physics of a single drop, how it behaves, how its shape tells us about angle and direction, all the way to these complex patterns, passive, transfer, projected, that reveal velocity,

mechanism, sequence, and even positioning.

That's the essence of it.

It's using that micro -level physics and macro -level geometry, applying math, like that simple sine function, to reconstruct dynamic, often violent events.

It really highlights how critical, careful observation and measurement are at the scene.

Taking something that looks chaotic and finding the underlying order using these principles.

It truly is.

Precision at every step from finding the stain to calculating the trajectory is paramount.

It makes you think, though.

We talked about surface texture and angle, but what about other factors, like how fast blood clots or how different fabrics absorb blood versus letting it spatter?

How could one of those maybe less obvious environmental things completely change how someone interprets, say, a perimeter stain, or how accurate that final trajectory calculation really is?

That's a really sharp point.

Those subtle environmental variables or the specific properties of the target material beyond just rough or smooth can introduce complieties.

It underscores why experience and rigorous methodology are so vital in this field.

There's always more nuance.

Definitely something for us all to think about.

Well, this has been an incredibly insightful deep dive.

Thank you so much for breaking down blood stain pattern analysis for us.

My pleasure.

It's fascinating stuff.

Indeed it is.

Thanks everyone for joining us and we'll see you on the next deep dive.